Fix: use correct return type in Javadoc documentation (#6344)

Author: lorenzpahl

src/main/java/io/reactivex/Flowable.java

@@ -9289,7 +9289,7 @@ public final Maybe<T> elementAt(long index) {
     }
 
     /**
-     * Returns a Flowable that emits the item found at a specified index in a sequence of emissions from
+     * Returns a Single that emits the item found at a specified index in a sequence of emissions from
      * this Flowable, or a default item if that index is out of range.
      * <p>
      * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/elementAtOrDefault.png" alt="">
@@ -9305,7 +9305,7 @@ public final Maybe<T> elementAt(long index) {
      *            the zero-based index of the item to retrieve
      * @param defaultItem
      *            the default item
-     * @return a Flowable that emits the item at the specified position in the sequence emitted by the source
+     * @return a Single that emits the item at the specified position in the sequence emitted by the source
      *         Publisher, or the default item if that index is outside the bounds of the source sequence
      * @throws IndexOutOfBoundsException
      *             if {@code index} is less than 0
@@ -9323,7 +9323,7 @@ public final Single<T> elementAt(long index, T defaultItem) {
     }
 
     /**
-     * Returns a Flowable that emits the item found at a specified index in a sequence of emissions from
+     * Returns a Single that emits the item found at a specified index in a sequence of emissions from
      * this Flowable or signals a {@link NoSuchElementException} if this Flowable has fewer elements than index.
      * <p>
      * <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/elementAtOrDefault.png" alt="">
@@ -9337,7 +9337,7 @@ public final Single<T> elementAt(long index, T defaultItem) {
      *
      * @param index
      *            the zero-based index of the item to retrieve
-     * @return a Flowable that emits the item at the specified position in the sequence emitted by the source
+     * @return a Single that emits the item at the specified position in the sequence emitted by the source
      *         Publisher, or the default item if that index is outside the bounds of the source sequence
      * @throws IndexOutOfBoundsException
      *             if {@code index} is less than 0

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

@@ -9507,7 +9507,7 @@ public final Maybe<T> lastElement() {
      *
      * @param defaultItem
      *            the default item to emit if the source ObservableSource is empty
-     * @return an Observable that emits only the last item emitted by the source ObservableSource, or a default item
+     * @return a Single that emits only the last item emitted by the source ObservableSource, or a default item
      *         if the source ObservableSource is empty
      * @see <a href="http://reactivex.io/documentation/operators/last.html">ReactiveX operators documentation: Last</a>
      */