DefaultObserver javadoc example fix

[devisnik]

The example in the javadoc of DefaultObserver is not compiling, DefaultObserver is not a Disposable.
This PR updates the javadoc to provide a compiling example.

[codecov]

Codecov Report

Merging #5405 into 2.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5405      +/-   ##
===========================================
+ Coverage     96.12%   96.2%   +0.08%     
- Complexity     5783    5785       +2     
===========================================
  Files           630     630              
  Lines         41197   41197              
  Branches       5728    5728              
===========================================
+ Hits          39602   39635      +33     
+ Misses          624     611      -13     
+ Partials        971     951      -20

Impacted Files	Coverage Δ	Complexity Δ
...n/java/io/reactivex/observers/DefaultObserver.java	100% <ø> (ø)	5 <0> (ø)	⬇️
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	95.58% <0%> (-2.95%)	2% <0%> (ø)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <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%)
...ternal/operators/flowable/FlowableSampleTimed.java	95.58% <0%> (-1.48%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	92.51% <0%> (-1.33%)	9% <0%> (ø)
... and 33 more

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[akarnokd]

Thanks for contributing. DefaultSubscriber's documentation has the same problem. Would you like to post a fix for that too?

[devisnik]

Sorry, just saw your comment. I'll post a fix for that as well.