Fixed Javadoc for Flowable and Observable reduce.

[leonardortlima]

Fixed Javadoc for Flowable.reduce(T, BiFunction) and Observable.reduce(T, BiFunction) mentioning the wrong return type.

#5456

[codecov]

Codecov Report

Merging #5476 into 2.x will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5476      +/-   ##
============================================
- Coverage     96.14%   96.06%   -0.09%     
+ Complexity     5801     5798       -3     
============================================
  Files           631      631              
  Lines         41297    41297              
  Branches       5743     5743              
============================================
- Hits          39704    39670      -34     
- Misses          621      639      +18     
- Partials        972      988      +16

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	523 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	85.93% <0%> (-6.78%)	56% <0%> (-1%)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <0%> (-6.13%)	2% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-4.92%)	2% <0%> (ø)
...al/operators/observable/ObservableSampleTimed.java	95% <0%> (-3.34%)	3% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	87.58% <0%> (-3.27%)	2% <0%> (ø)
...ternal/operators/completable/CompletableCache.java	96.96% <0%> (-3.04%)	23% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
... and 37 more

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