2.x: Fix some grammar mistakes

[akarnokd]

This PR fixes a bunch of typos (in many files) and grammar mistakes in Flowable (based on Grammarly checks).

The PR also fixes the JavadocWording checks to start from the beginning of the method's javadoc as well as checking for cross-line a/an typos.

[akarnokd]

FYI, I was using this little program to extract and scrape the javadoc, split it into multiple files and feed them one by one to Grammarly's desktop app. Unfortunately, only basic mistakes are highlighted and other more complicated errors (such as passive misuse, wording) require paid services.

[codecov]

Codecov Report

Merging #5959 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5959      +/-   ##
============================================
+ Coverage     98.23%   98.24%   +<.01%     
  Complexity     6016     6016              
============================================
  Files           656      656              
  Lines         44036    44036              
  Branches       6100     6100              
============================================
+ Hits          43259    43262       +3     
+ Misses          238      233       -5     
- Partials        539      541       +2

Impacted Files	Coverage Δ	Complexity Δ
...perators/mixed/ObservableConcatMapCompletable.java	100% <ø> (+2.25%)	3 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	115 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	142 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	557 <0> (ø)	⬇️
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <ø> (-0.86%)	2 <0> (ø)
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	535 <0> (ø)	⬇️
.../java/io/reactivex/processors/ReplayProcessor.java	99.58% <ø> (ø)	52 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.23%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-4.58%)	2% <0%> (ø)
... and 25 more

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