Updating JavaDoc with correct return types

[levaja]

A small PR with updated JavaDoc, mentioned in the issue #5595 (thanks to @RobLewis for reporting it).

Updated documentation about return types, where Observable was incorrectly mentioned instead of Single.

[akarnokd]

Great, thanks!

[akarnokd]

The Flowable variants are okay.

[codecov]

Codecov Report

Merging #5600 into 2.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #5600      +/-   ##
===========================================
- Coverage     96.2%   96.15%   -0.06%     
- Complexity    5838     5840       +2     
===========================================
  Files          632      632              
  Lines        41473    41473              
  Branches      5745     5745              
===========================================
- Hits         39898    39877      -21     
- Misses         632      637       +5     
- Partials       943      959      +16

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-6.77%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-4.26%)	32% <0%> (-3%)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	93.28% <0%> (-2.99%)	2% <0%> (ø)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
... and 22 more

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