3.x: fix typo in BehaviorSubject.java

[harrygaoos]

Thank you for contributing to RxJava. Before pressing the "Create Pull Request" button, please consider the following points:

• Please give a description about what and why you are contributing, even if it's trivial.
  Fixed typo below:

 * observable.onNext(1);
 * // this will "clear" the cache
 * observable.onNext(EMPTY);

to

 * subject.onNext(1);
 * // this will "clear" the cache
 * subject.onNext(EMPTY);

observable has no #onNext() method.

• Please include the issue list number(s) or other PR numbers in the description if you are contributing in response to those.

• Please include a reasonable set of unit tests if you contribute new code or change an existing one. If you contribute an operator, (if applicable) please make sure you have tests for working with an empty, just, range of values as well as an error source, with and/or without backpressure and see if unsubscription/cancellation propagates correctly.

[codecov]

Codecov Report

Merging #7452 (39438c1) into 3.x (f980960) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7452      +/-   ##
============================================
+ Coverage     99.53%   99.54%   +0.01%     
+ Complexity     6782     6781       -1     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
+ Hits          47261    47267       +6     
+ Misses          101       97       -4     
+ Partials        120      118       -2     

Impacted Files	Coverage Δ
...io/reactivex/rxjava3/subjects/BehaviorSubject.java	100.00% <ø> (ø)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	⬇️
...va/io/reactivex/rxjava3/subjects/AsyncSubject.java	97.87% <0.00%> (-2.13%)	⬇️
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	95.23% <0.00%> (-0.80%)	⬇️
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	98.96% <0.00%> (-0.21%)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	99.35% <0.00%> (+0.32%)	⬆️
...a3/internal/operators/flowable/FlowableReplay.java	99.78% <0.00%> (+0.85%)	⬆️
...3/internal/operators/flowable/FlowablePublish.java	100.00% <0.00%> (+0.99%)	⬆️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	97.92% <0.00%> (+1.55%)	⬆️
... and 1 more

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[akarnokd]

Thanks!