Correct method name for Flowable-->Observable. (#4127)

Author: JakeWharton

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

@@ -3478,7 +3478,7 @@ public Collection<V> apply(K k) {
 
     @BackpressureSupport(BackpressureKind.NONE)
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final Observable<T> toNbpObservable() {
+    public final Observable<T> toObservable() {
         return Observable.fromPublisher(this);
     }