Remove apostrophe to correct grammar.

[eventualbuddha]

• Please give a description about what and why you are contributing, even if it's trivial.

This pull request corrects a minor grammar issue in the README. In this case we want the 3rd person present form of "let", not the contraction "let us".

• Please include the issue list number(s) or other PR numbers in the description if you are contributing in response to those.

n/a

• Please include a reasonable set of unit tests if you contribute new code or change an existing one. If you contribute an operator, (if applicable) please make sure you have tests for working with an empty, just, range of values as well as an error source, with and/or without backpressure and see if unsubscription/cancellation propagates correctly.

n/a

[codecov]

Codecov Report

Merging #5793 into 2.x will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5793      +/-   ##
============================================
- Coverage     96.29%   96.25%   -0.04%     
- Complexity     5809     5810       +1     
============================================
  Files           634      634              
  Lines         41607    41607              
  Branches       5770     5770              
============================================
- Hits          40066    40050      -16     
- Misses          614      623       +9     
- Partials        927      934       +7

Impacted Files	Coverage Δ	Complexity Δ
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.24% <0%> (-5.83%)	54% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-2.64%)	11% <0%> (+1%)
...nternal/operators/parallel/ParallelSortedJoin.java	92.75% <0%> (-2.18%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	91.97% <0%> (-2.14%)	5% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
... and 23 more

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