Fix JavaDocs of Single.doOnTerminate refer to onComplete notification

[SupasinTatiyanupanwong]

As mentioned in JavaDocs of this class that Single does not have "onComplete" notification.
This newly introduced method refer to that notification, which looks to me it should be onSuccess instead.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.x@5c83b50). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             2.x    #6565   +/-   ##
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  Coverage       ?   98.24%           
  Complexity     ?     6333           
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  Files          ?      677           
  Lines          ?    45488           
  Branches       ?     6321           
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  Hits           ?    44691           
  Misses         ?      244           
  Partials       ?      553

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	149 <0> (?)

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