Fix linting in firebase-database

Author: daymxn

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

@@ -256,6 +256,7 @@ public String getKey() {
   /**
    * Gives access to all of the immediate children of this snapshot. Can be used in native for
    * loops:
+   *
    * <pre>
    * for (DataSnapshot child : parent.getChildren()) {
    *   &nbsp;&nbsp;&nbsp;&nbsp;...

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

@@ -92,6 +92,7 @@ public long getChildrenCount() {
 
   /**
    * Used to iterate over the immediate children at this location
+   *
    * <pre>
    *     for (MutableData child : parent.getChildren()) {
    *         &nbsp;&nbsp;&nbsp;&nbsp;...