2.x: Fix Javadoc of Flowable.toObservable

[akarnokd]

The Flowable.toObservable() javadoc was referring to Publisher instead of Observable.

[codecov]

Codecov Report

Merging #5309 into 2.x will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5309      +/-   ##
============================================
+ Coverage     95.97%   96.07%   +0.09%     
- Complexity     5762     5769       +7     
============================================
  Files           630      630              
  Lines         41146    41146              
  Branches       5714     5714              
============================================
+ Hits          39488    39529      +41     
+ Misses          685      653      -32     
+ Partials        973      964       -9

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	99.73% <ø> (ø)	520 <0> (ø)	⬇️
...vex/internal/operators/single/SingleTakeUntil.java	88.52% <0%> (-4.92%)	2% <0%> (ø)
...ors/observable/ObservableSampleWithObservable.java	93.97% <0%> (-4.82%)	3% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-4.48%)	2% <0%> (ø)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <0%> (-2.78%)	2% <0%> (ø)
.../operators/observable/ObservableTakeLastTimed.java	96% <0%> (-2.67%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	93.04% <0%> (-2.14%)	5% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	91.5% <0%> (-1.97%)	2% <0%> (ø)
... and 41 more

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