3.x: Update Readme.md with the new package locations

[akarnokd]

Adjust the README.md with the new package locations.

[codecov]

Codecov Report

Merging #6631 into 3.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6631      +/-   ##
============================================
- Coverage     98.17%   98.14%   -0.03%     
+ Complexity     6189     6187       -2     
============================================
  Files           678      678              
  Lines         44804    44804              
  Branches       6177     6177              
============================================
- Hits          43986    43975      -11     
- Misses          282      288       +6     
- Partials        536      541       +5

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	84.02% <0%> (-11.81%)	2% <0%> (ø)
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...internal/operators/flowable/FlowableSwitchMap.java	93.92% <0%> (-1.41%)	3% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableReplay.java	91.97% <0%> (-0.83%)	19% <0%> (ø)
...x/rxjava3/internal/queue/SpscLinkedArrayQueue.java	96.82% <0%> (-0.8%)	38% <0%> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	94.89% <0%> (-0.73%)	2% <0%> (ø)
... and 18 more

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