Fix 'heading used out of sequence' JavaDoc error (#2896)

Author: pivovarit

vavr/generator/Generator.scala

@@ -1373,7 +1373,7 @@ def generateMainClasses(): Unit = {
          * import static io.vavr.API.*;
          * </code></pre>
          *
-         * <h3>For-comprehension</h3>
+         * <h2>For-comprehension</h2>
          * <p>
          * The {@code For}-comprehension is syntactic sugar for nested for-loops. We write
          *

vavr/src-gen/main/java/io/vavr/API.java

@@ -48,7 +48,7 @@
  * import static io.vavr.API.*;
  * </code></pre>
  *
- * <h3>For-comprehension</h3>
+ * <h2>For-comprehension</h2>
  * <p>
  * The {@code For}-comprehension is syntactic sugar for nested for-loops. We write
  *

vavr/src/main/java/io/vavr/concurrent/Promise.java

@@ -35,7 +35,7 @@
  * The underlying {@code Executor} is used to execute asynchronous handlers, e.g. via
  * {@code promise.future().onComplete(...)}.
  *
- * <h3>Creation</h3>
+ * <h2>Creation</h2>
  * <p>
  * Promise offers static factory methods to create new promises which hasn't been fulfilled yet:
  * <ul>