2.x: Javadocs: add period to custom scheduler use sentences

[akarnokd]

There were missing . from sentences in the operator Javadocs detailing certain overloads use custom Schedulers.

[codecov]

Codecov Report

Merging #5717 into 2.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5717      +/-   ##
============================================
- Coverage     96.33%   96.27%   -0.07%     
+ Complexity     5820     5819       -1     
============================================
  Files           634      634              
  Lines         41604    41604              
  Branches       5761     5761              
============================================
- Hits          40079    40053      -26     
- Misses          600      618      +18     
- Partials        925      933       +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	508 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	135 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	169 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	525 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	91.15% <0%> (-3.99%)	62% <0%> (+1%)
...l/operators/observable/ObservableFlatMapMaybe.java	90.84% <0%> (-2.62%)	2% <0%> (ø)
...java/io/reactivex/subjects/CompletableSubject.java	97.4% <0%> (-2.6%)	36% <0%> (-1%)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-1.45%)	2% <0%> (ø)
... and 20 more

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