3.x: Javadoc: Fix wording of *OnSubscribe interfaces

[akarnokd]

Bad wording: "an instance of a(n) X instance".

[codecov]

Codecov Report

Merging #7274 (1ce19fc) into 3.x (3ba64ce) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@ Coverage Diff @@## 3.x #7274 +/- ## ============================================ - Coverage 99.56% 99.53% -0.03%  Complexity 6757 6757 ============================================ Files 747 747 Lines 47391 47391 Branches 6382 6382 ============================================ - Hits 47183 47172 -11 - Misses 92 100 +8 - Partials 116 119 +3 

Impacted Files	Coverage Δ	Complexity Δ
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	50.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	96.51% <0.00%> (-1.17%)	2.00% <0.00%> (ø%)
...3/internal/operators/flowable/FlowablePublish.java	99.00% <0.00%> (-1.00%)	18.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableReplay.java	98.92% <0.00%> (-0.86%)	21.00% <0.00%> (ø%)
...l/operators/observable/ObservableFlatMapMaybe.java	97.88% <0.00%> (-0.71%)	2.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	96.37% <0.00%> (-0.52%)	2.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	97.87% <0.00%> (-0.36%)	3.00% <0.00%> (ø%)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.58% <0.00%> (+0.62%)	49.00% <0.00%> (ø%)
...3/internal/operators/observable/ObservableZip.java	100.00% <0.00%> (+0.68%)	6.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (+0.79%)	2.00% <0.00%> (ø%)

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