2.x: Fix Javadoc of FunctionX

[bruce3x]

fix the mistake for FunctionX javadoc

[codecov]

Codecov Report

Merging #5347 into 2.x will decrease coverage by 0.21%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5347      +/-   ##
============================================
- Coverage     96.21%   95.99%   -0.22%     
+ Complexity     5785     5773      -12     
============================================
  Files           630      630              
  Lines         41162    41162              
  Branches       5721     5721              
============================================
- Hits          39603    39514      -89     
- Misses          608      657      +49     
- Partials        951      991      +40

Impacted Files	Coverage Δ	Complexity Δ
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-5.68%)	31% <0%> (-4%)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-5.09%)	10% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	87.61% <0%> (-4.87%)	61% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-4.39%)	10% <0%> (ø)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ors/observable/ObservableSampleWithObservable.java	93.97% <0%> (-3.62%)	3% <0%> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
... and 35 more

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