Fixed typos for comments

[lxsmnsyc]

I was looking at the docs and found that some details does not match the given code.

For example:

It says 'new Completable instance' when, in fact, it returns a Maybe.

[codecov]

Codecov Report

Merging #6453 into 2.x will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6453      +/-   ##
============================================
- Coverage     98.27%   98.23%   -0.04%     
  Complexity     6296     6296              
============================================
  Files           675      675              
  Lines         45173    45173              
  Branches       6246     6246              
============================================
- Hits          44393    44376      -17     
- Misses          241      251      +10     
- Partials        539      546       +7

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	173 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	149 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-5.23%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-3.87%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	95.21% <0%> (-1.07%)	3% <0%> (ø)
... and 21 more

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