Make Observable.doOnCompleted name match Completable.doOnComplete in 1.x

[zach-klippenstein]

Observable has a doOnCompleted method (past tense), and Completable has doOnComplete (present tense). The 2.x branch changes Observable's method to be present tense. The 1.x branch can add alias the past tense method in Observable and deprecate it.

I'm happy to make the change if this is approved.

[akarnokd]

Sure.