2.x: Fix copy-paste errors in SingleSubject JavaDoc

[akarnokd]

The SingleSubject JavaDoc was derived from the MaybeSubject JavaDoc and thus some mentions of onComplete remained in there. This PR fixes copy-paste errors.

[codecov]

Codecov Report

Merging #5834 into 2.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5834      +/-   ##
============================================
- Coverage     96.42%   96.35%   -0.07%     
+ Complexity     5816     5814       -2     
============================================
  Files           634      634              
  Lines         41761    41761              
  Branches       5796     5796              
============================================
- Hits          40268    40240      -28     
- Misses          578      593      +15     
- Partials        915      928      +13

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/reactivex/subjects/SingleSubject.java	100% <ø> (ø)	38 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	87.58% <0%> (-7.85%)	2% <0%> (ø)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	91.45% <0%> (-5.13%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	90.13% <0%> (-4.94%)	59% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-4.39%)	10% <0%> (-1%)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	93.83% <0%> (-1.77%)	10% <0%> (ø)
... and 21 more

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