Fix broken code elements in docs (#4292)

* Fix code blocks

* Fix linting in firebase-database

* Fix FirebaseMLDownloader linting

Author: daymxn

firebase-database/src/main/java/com/google/firebase/database/DataSnapshot.java

@@ -166,7 +166,7 @@ public Object getValue(boolean useExportFormat) {
    *
    * An example class might look like:
    *
-   * <pre><code>
+   * <pre>
    *     class Message {
    *         private String author;
    *         private String text;
@@ -190,7 +190,7 @@ public Object getValue(boolean useExportFormat) {
    *
    *     // Later
    *     Message m = snapshot.getValue(Message.class);
-   * </code></pre>
+   * </pre>
    *
    * @param valueType The class into which this snapshot should be marshalled
    * @param <T> The type to return. Implicitly defined from the class passed in
@@ -208,10 +208,10 @@ public <T> T getValue(@NonNull Class<T> valueType) {
    * properly-typed Collection. So, in the case where you want a {@link java.util.List} of Message
    * instances, you will need to do something like the following:
    *
-   * <pre><code>
+   * <pre>
    *     GenericTypeIndicator&lt;List&lt;Message&gt;&gt; t = new GenericTypeIndicator&lt;List&lt;Message&gt;&gt;() {};
    *     List&lt;Message&gt; messages = snapshot.getValue(t);
-   * </code></pre>
+   * </pre>
    *
    * It is important to use a subclass of {@link GenericTypeIndicator}. See {@link
    * GenericTypeIndicator} for more details
@@ -255,11 +255,13 @@ public String getKey() {
 
   /**
    * Gives access to all of the immediate children of this snapshot. Can be used in native for
-   * loops: <code>
-   * <br>    for (DataSnapshot child : parent.getChildren()) {
-   * <br>    &nbsp;&nbsp;&nbsp;&nbsp;...
-   * <br>    }
-   * </code>
+   * loops:
+   *
+   * <pre>
+   * for (DataSnapshot child : parent.getChildren()) {
+   *   &nbsp;&nbsp;&nbsp;&nbsp;...
+   * }
+   * </pre>
    *
    * @return The immediate children of this snapshot
    */

firebase-database/src/main/java/com/google/firebase/database/GenericTypeIndicator.java

@@ -25,7 +25,7 @@
  * {@link DataSnapshot}: <br>
  * <br>
  *
- * <pre><code>
+ * <pre>
  *     class Message {
  *         private String author;
  *         private String text;
@@ -51,7 +51,7 @@
  *     GenericTypeIndicator&lt;List&lt;Message&gt;&gt; t = new GenericTypeIndicator&lt;List&lt;Message&gt;&gt;() {};
  *     List&lt;Message&gt; messages = snapshot.getValue(t);
  *
- * </code></pre>
+ * </pre>
  *
  * @param <T> The type of generic collection that this instance servers as an indicator for
  */

firebase-database/src/main/java/com/google/firebase/database/MutableData.java

@@ -91,11 +91,13 @@ public long getChildrenCount() {
   }
 
   /**
-   * Used to iterate over the immediate children at this location <code>
-   *     <br>for (MutableData child : parent.getChildren()) {
-   *         <br>&nbsp;&nbsp;&nbsp;&nbsp;...
-   *     <br>}
-   * </code>
+   * Used to iterate over the immediate children at this location
+   *
+   * <pre>
+   *     for (MutableData child : parent.getChildren()) {
+   *         &nbsp;&nbsp;&nbsp;&nbsp;...
+   *     }
+   * </pre>
    *
    * @return The immediate children at this location
    */
@@ -196,7 +198,7 @@ public Object getValue() {
    *
    * An example class might look like:
    *
-   * <pre><code>
+   * <pre>
    *     class Message {
    *         private String author;
    *         private String text;
@@ -220,7 +222,7 @@ public Object getValue() {
    *
    *     // Later
    *     Message m = mutableData.getValue(Message.class);
-   * </code></pre>
+   * </pre>
    *
    * @param valueType The class into which this data in this instance should be marshalled
    * @param <T> The type to return. Implicitly defined from the class passed in
@@ -238,11 +240,11 @@ public <T> T getValue(@NonNull Class<T> valueType) {
    * properly-typed Collection. So, in the case where you want a {@link java.util.List} of Message
    * instances, you will need to do something like the following:
    *
-   * <pre><code>
+   * <pre>
    *     GenericTypeIndicator&lt;List&lt;Message&gt;&gt; t =
    *         new GenericTypeIndicator&lt;List&lt;Message&gt;&gt;() {};
    *     List&lt;Message&gt; messages = mutableData.getValue(t);
-   * </code></pre>
+   * </pre>
    *
    * It is important to use a subclass of {@link GenericTypeIndicator}. See {@link
    * GenericTypeIndicator} for more details