Update DESIGN.md

[fjoshuajr]

I've just read the DESIGN.md and noticed some things that I could do to improve the quality of the DESIGN.md. So as a result of my "proofreading" I mainly:

• Added periods at the ending of some sentences.
• Did case matching of certain types and terms. e.g. OnSubscribe -> onSubscribe OR flowable -> Flowable.

Hope it helps! 😄

[codecov]

Codecov Report

Merging #6033 into 2.x will increase coverage by 0.1%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x    #6033     +/-   ##
===========================================
+ Coverage     98.25%   98.35%   +0.1%     
  Complexity     6171     6171             
===========================================
  Files           665      665             
  Lines         44729    44729             
  Branches       6205     6205             
===========================================
+ Hits          43949    43995     +46     
+ Misses          237      213     -24     
+ Partials        543      521     -22

Impacted Files	Coverage Δ	Complexity Δ
.../operators/mixed/FlowableConcatMapCompletable.java	97.43% <0%> (-2.57%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	97.61% <0%> (-1.2%)	2% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	94.89% <0%> (-0.73%)	2% <0%> (ø)
...va/io/reactivex/processors/MulticastProcessor.java	98.23% <0%> (-0.45%)	83% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.23% <0%> (ø)	2% <0%> (ø)	⬇️
... and 17 more

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[akarnokd]

Great, I hope you take a look at other documentation as well!

[fjoshuajr]

Thank you! 😃