2.x: fix DisposableX copy-paste error in javadoc

Author: akarnokd

src/main/java/io/reactivex/observers/DisposableObserver.java

@@ -27,7 +27,7 @@
  * <p>Use the public {@link #dispose()} method to dispose the sequence from within an
  * {@code onNext} implementation.
  *
- * <p>Like all other consumers, {@code DefaultObserver} can be subscribed only once.
+ * <p>Like all other consumers, {@code DisposableObserver} can be subscribed only once.
  * Any subsequent attempt to subscribe it to a new source will yield an
  * {@link IllegalStateException} with message {@code "Disposable already set!"}.
  *

src/main/java/io/reactivex/subscribers/DisposableSubscriber.java

@@ -37,7 +37,7 @@
  * Calling {@link #request(long)} inside {@link #onNext(Object)} can happen at any time
  * because by design, {@code onNext} calls from upstream are non-reentrant and non-overlapping.
  *
- * <p>Like all other consumers, {@code DefaultSubscriber} can be subscribed only once.
+ * <p>Like all other consumers, {@code DisposableSubscriber} can be subscribed only once.
  * Any subsequent attempt to subscribe it to a new source will yield an
  * {@link IllegalStateException} with message {@code "Subscription already set!"}.
  *