Update Additional-Reading.md

[khkong]

#6132

• First line was edited.
• 'What is Reactive Programming?' was wrong link, so deleted.

[akarnokd]

The site went down but managed to track down the video:

https://www.youtube.com/watch?v=-8Y1-lE6NSA

The original blog did not feature much text so it can be safely ignored.
Please update the PR accordingly.

[codecov]

Codecov Report

Merging #6496 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6496      +/-   ##
============================================
- Coverage     98.23%   98.21%   -0.03%     
  Complexity     6296     6296              
============================================
  Files           675      675              
  Lines         45163    45163              
  Branches       6244     6244              
============================================
- Hits          44368    44357      -11     
- Misses          256      259       +3     
- Partials        539      547       +8

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/observable/ObservableCreate.java	93.16% <0%> (-5.13%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-2.18%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	97.61% <0%> (-1.2%)	2% <0%> (ø)
...activex/internal/schedulers/ExecutorScheduler.java	95.97% <0%> (-1.01%)	10% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	97.34% <0%> (-0.89%)	10% <0%> (-1%)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	97.72% <0%> (-0.76%)	37% <0%> (-1%)
... and 14 more

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[khkong]

Yes, I will add it.