Fix links for Single class

[apodkutin]

• Change observable.html to single.html
• Delete completable.html link which doesn't exist

[codecov]

Codecov Report

Merging #6066 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6066      +/-   ##
============================================
+ Coverage     98.26%   98.27%   +0.01%     
+ Complexity     6194     6189       -5     
============================================
  Files           666      666              
  Lines         44806    44806              
  Branches       6206     6206              
============================================
+ Hits          44029    44034       +5     
+ Misses          239      232       -7     
- Partials        538      540       +2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	93.08% <0%> (-3.2%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-1.94%)	2% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
... and 24 more

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