Fix javadoc for Observable.reduce() and Observable.reduceWith()

[devisnik]

The sample code given in the javadoc for Observable.reduce() isn't compiling, since the function returns a Single.
This PR changes it to a compiling example.
It also updates the javadoc for reduceWith() to better match its actual behaviour.

Remark
I'm not sure whether the sample code for reduce() is needed at all. The problem that it addresses might be solved better by using reduceWith(), imho.

Update

• similar changes applied to Flowable.reduce() and Flowable.reduceWith()

[akarnokd]

Could you also fix Flowable's javadoc?

[devisnik]

@akarnokd Done.
What do you think about referring to reduceWith() instead of giving the example that uses defer()?

[akarnokd]

The defer example should stay but you can add @see and a 3rd example.

[akarnokd]

A small original mention of Publisher could be fixed.

[akarnokd]

This should be Flowable.defer.

[devisnik]

Updated according to the feedback.

[akarnokd]

Great!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.x@c11f715). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             2.x    #5406   +/-   ##
======================================
  Coverage       ?   96.19%           
  Complexity     ?     5789           
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  Files          ?      630           
  Lines          ?    41197           
  Branches       ?     5728           
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  Hits           ?    39629           
  Misses         ?      605           
  Partials       ?      963

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (?)
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	523 <0> (?)

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