Observable's javadoc cleanup

[anthonyms]

The sample code in the Observable javadoc erroneously uses onNext(Integer t) for a DisposableObserver<String>

This has been corrected to be onNext(String t)

[codecov]

Codecov Report

Merging #5992 into 2.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5992      +/-   ##
============================================
- Coverage     98.28%   98.22%   -0.06%     
+ Complexity     6054     6051       -3     
============================================
  Files           656      656              
  Lines         44078    44078              
  Branches       6118     6118              
============================================
- Hits          43321    43297      -24     
- Misses          229      234       +5     
- Partials        528      547      +19

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	535 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.98%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	97.02% <0%> (-1.71%)	11% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...ex/internal/operators/flowable/FlowableCreate.java	96.42% <0%> (-1.3%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
... and 19 more

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