Fix Javadoc for Maybe.toSingle

[paulblessing]

The Javadoc for the Maybe.toSingle() overload that accepts a defaultValue incorrectly states that it will convert an empty Maybe into a Single that signals NoSuchElementException (likely copied from the version of Maybe.toSingle() that does not accept a default value).

This change updates the Javadoc to indicate that the returned Single will emit defaultValue if called on an empty Maybe.

Relevant existing test:

RxJava/src/test/java/io/reactivex/maybe/MaybeTest.java

Line 2730 in 8819cc9

Maybe.empty().toSingle(100)

[codecov]

Codecov Report

Merging #5343 into 2.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5343      +/-   ##
============================================
+ Coverage     96.14%   96.23%   +0.08%     
- Complexity     5780     5784       +4     
============================================
  Files           630      630              
  Lines         41146    41146              
  Branches       5714     5714              
============================================
+ Hits          39561    39597      +36     
+ Misses          623      603      -20     
+ Partials        962      946      -16

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	168 <0> (ø)	⬇️
...vex/internal/operators/single/SingleTakeUntil.java	88.52% <0%> (-6.56%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	88.49% <0%> (-4.87%)	62% <0%> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-3.51%)	9% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
.../main/java/io/reactivex/subjects/AsyncSubject.java	97.41% <0%> (-1.73%)	55% <0%> (-1%)
...n/java/io/reactivex/processors/AsyncProcessor.java	97.45% <0%> (-1.7%)	55% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
... and 28 more

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