Fix Observable javadoc (#5944)

[RomanWuattier]

• Replace doOnCancel by doOnDispose in Observable javadoc

Thank you for contributing to RxJava. Before pressing the "Create Pull Request" button, please consider the following points:

• Please give a description about what and why you are contributing, even if it's trivial.

Replace doOnCancel by doOnDispose in Observable javadoc

• Please include the issue list number(s) or other PR numbers in the description if you are contributing in response to those.

#5944

• Please include a reasonable set of unit tests if you contribute new code or change an existing one. If you contribute an operator, (if applicable) please make sure you have tests for working with an empty, just, range of values as well as an error source, with and/or without backpressure and see if unsubscription/cancellation propagates correctly.

Javadoc only

[codecov]

Codecov Report

Merging #5948 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5948      +/-   ##
============================================
+ Coverage     98.25%   98.28%   +0.03%     
- Complexity     6018     6020       +2     
============================================
  Files           656      656              
  Lines         44036    44036              
  Branches       6100     6100              
============================================
+ Hits          43266    43281      +15     
+ Misses          230      221       -9     
+ Partials        540      534       -6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	535 <0> (ø)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-1.99%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-1.64%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	93.08% <0%> (-1.6%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.2% <0%> (-0.97%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	86.26% <0%> (-0.96%)	3% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	99.11% <0%> (-0.89%)	11% <0%> (ø)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0%> (-0.86%)	2% <0%> (ø)
... and 22 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 2770e4d...c883e2b. Read the comment docs.