Observable javadoc cleanup (#5992)

The sample code in the Observable javadoc erroneously uses onNext(Integer t) for a DisposableObserver<String>
This has been corrected to be onNext(String t)

Author: Anthony Musyoki

src/main/java/io/reactivex/Observable.java

@@ -74,7 +74,7 @@
  *         @Override public void onStart() {
  *             System.out.println("Start!");
  *         }
- *         @Override public void onNext(Integer t) {
+ *         @Override public void onNext(String t) {
  *             System.out.println(t);
  *         }
  *         @Override public void onError(Throwable t) {