Fix JavaDoc image for Single#flatMapObservable

[veyndan]

Single.flatMapObservable.png should be used for Single#flatMapObservable instead of Single.flatMap.png.

[codecov]

Codecov Report

Merging #5297 into 2.x will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5297      +/-   ##
============================================
+ Coverage     96.01%   96.11%   +0.09%     
- Complexity     5751     5758       +7     
============================================
  Files           628      628              
  Lines         41085    41085              
  Branches       5703     5703              
============================================
+ Hits          39449    39488      +39     
+ Misses          655      631      -24     
+ Partials        981      966      -15

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	99.32% <ø> (ø)	131 <0> (ø)	⬇️
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...ava/io/reactivex/internal/util/HalfSerializer.java	97.5% <0%> (-2.5%)	22% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...operators/observable/ObservableConcatMapEager.java	97.26% <0%> (-2.19%)	2% <0%> (ø)
...reactivex/internal/operators/maybe/MaybeUsing.java	93.93% <0%> (-2.03%)	4% <0%> (ø)
.../main/java/io/reactivex/subjects/AsyncSubject.java	97.41% <0%> (-1.73%)	55% <0%> (-1%)
... and 38 more

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[akarnokd]

Thanks!