Fixing javadoc's code example of Observable#lift.

[lcybo]

Code example in javadoc for Observable#lift seems not compilable in 2.x.
Also, parameter name of ObservableOperator#apply should be downstream I think.

[codecov]

Codecov Report

Merging #6104 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6104      +/-   ##
============================================
- Coverage     98.29%   98.29%   -0.01%     
+ Complexity     6198     6195       -3     
============================================
  Files           667      667              
  Lines         44862    44862              
  Branches       6213     6213              
============================================
- Hits          44097    44096       -1     
- Misses          228      229       +1     
  Partials        537      537

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	539 <0> (ø)	⬇️
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	96.61% <0%> (-3.39%)	10% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	95.21% <0%> (-1.07%)	3% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	86.9% <0%> (-0.96%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
... and 17 more

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