Correct documented return type of Single#flatMapObservable Function argument

[veyndan]

The argument of Single#flatMapObservable clearly states that it is expecting a Function which returns an ObservableSource but the documentation contradicts that.

[akarnokd]

Thanks!

[codecov]

Codecov Report

Merging #5304 into 2.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5304      +/-   ##
============================================
- Coverage     96.12%   96.06%   -0.07%     
+ Complexity     5755     5751       -4     
============================================
  Files           629      629              
  Lines         41112    41112              
  Branches       5705     5705              
============================================
- Hits          39520    39493      -27     
- Misses          628      648      +20     
- Partials        964      971       +7

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	99.33% <ø> (ø)	132 <0> (ø)	⬇️
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	85.93% <0%> (-5.73%)	57% <0%> (+1%)
.../java/io/reactivex/subjects/SerializedSubject.java	88.42% <0%> (-5.27%)	24% <0%> (-2%)
...java/io/reactivex/subjects/CompletableSubject.java	94.8% <0%> (-5.2%)	35% <0%> (-2%)
...ors/observable/ObservableSampleWithObservable.java	92.77% <0%> (-4.82%)	3% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	96.19% <0%> (-2.72%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	93.39% <0%> (-2.21%)	9% <0%> (ø)
... and 45 more

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