Fix Javadoc for Flowable and Observable reduce. (#5476)

Author: leonardortlima

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

@@ -10745,7 +10745,7 @@ public final Maybe<T> reduce(BiFunction<T, T, T> reducer) {
     }
 
     /**
-     * Returns a Flowable that applies a specified accumulator function to the first item emitted by a source
+     * Returns a Single that applies a specified accumulator function to the first item emitted by a source
      * Publisher and a specified seed value, then feeds the result of that function along with the second item
      * emitted by a Publisher into the same function, and so on until all items have been emitted by the
      * source Publisher, emitting the final result from the final call to your function as its sole item.

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

@@ -8988,7 +8988,7 @@ public final Maybe<T> reduce(BiFunction<T, T, T> reducer) {
     }
 
     /**
-     * Returns an Observable that applies a specified accumulator function to the first item emitted by a source
+     * Returns a Single that applies a specified accumulator function to the first item emitted by a source
      * ObservableSource and a specified seed value, then feeds the result of that function along with the second item
      * emitted by an ObservableSource into the same function, and so on until all items have been emitted by the
      * source ObservableSource, emitting the final result from the final call to your function as its sole item.