Fix terminology of cancel/dispose in the JavaDocs

[luis-cortes]

Resolves #6196

Changing terminology from cancel -> dispose wherever appropriate in the following classes:

• Observable.java
• Single.java
• Maybe.java
• Completable.java

Areas where I did not change from cancel -> disposal include:

• where the cancellation was in reference to a Flowable, Publisher, or a Future
• test() methods where the TestObserver returned actually has a cancel() method

[codecov]

Codecov Report

Merging #6199 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6199      +/-   ##
============================================
+ Coverage     98.23%   98.25%   +0.02%     
+ Complexity     6202     6201       -1     
============================================
  Files           667      667              
  Lines         44888    44888              
  Branches       6218     6218              
============================================
+ Hits          44095    44106      +11     
+ Misses          253      245       -8     
+ Partials        540      537       -3

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	117 <2> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	541 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	171 <4> (ø)	⬇️
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	95.21% <0%> (-1.07%)	3% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e0532b7...2eaba8a. Read the comment docs.

[akarnokd]

One small adjustment and its good to go.

[akarnokd]

"Note that if any of the observers to this Completable call dispose,"