3.x: Fix marbles of first(T)

[akarnokd]

The operator had its old name in the diagram. Also created a separate image for Flowable.

[codecov]

Codecov Report

Merging #7400 (c942c1d) into 3.x (9f91839) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7400      +/-   ##
============================================
- Coverage     99.53%   99.53%   -0.01%     
- Complexity     6777     6779       +2     
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47261    47259       -2     
+ Misses          102      100       -2     
- Partials        119      123       +4     

Impacted Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	94.36% <0.00%> (-3.53%)	⬇️
...nternal/operators/observable/ObservableCreate.java	95.72% <0.00%> (-3.42%)	⬇️
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	⬇️
...va/io/reactivex/rxjava3/subjects/AsyncSubject.java	97.87% <0.00%> (-2.13%)	⬇️
...nternal/operators/observable/ObservableWindow.java	98.21% <0.00%> (-1.79%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-1.59%)	⬇️
...operators/flowable/FlowableConcatMapScheduler.java	99.20% <0.00%> (-0.40%)	⬇️
... and 16 more

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