2.x: Fix JavaDoc wording of onTerminateDetach

[akarnokd]

A missing nulls word from Flowable.onTerminateDetach and Observable.onTerminateDetach JavaDoc. The other types have this word already.

Related: #5782.

[codecov]

Codecov Report

Merging #5783 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5783      +/-   ##
============================================
- Coverage     96.27%   96.25%   -0.02%     
- Complexity     5807     5808       +1     
============================================
  Files           634      634              
  Lines         41607    41607              
  Branches       5770     5770              
============================================
- Hits          40057    40049       -8     
- Misses          611      627      +16     
+ Partials        939      931       -8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	509 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	526 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	79.48% <0%> (-16.24%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-6.72%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.41% <0%> (-4.17%)	34% <0%> (-3%)
...ava/io/reactivex/processors/BehaviorProcessor.java	91.03% <0%> (-4.04%)	60% <0%> (+1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
... and 27 more

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