Remove unnecessary git command from build instructions

[tylerbwong]

Removing line to match wiki instructions and since 2.x is the default branch

[codecov]

Codecov Report

Merging #5572 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5572      +/-   ##
===========================================
- Coverage     96.21%   96.2%   -0.02%     
  Complexity     5824    5824              
===========================================
  Files           631     631              
  Lines         41421   41421              
  Branches       5739    5739              
===========================================
- Hits          39855   39849       -6     
- Misses          616     619       +3     
- Partials        950     953       +3

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableFlatMapSingle.java	89.55% <0%> (-5.98%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.97% <0%> (-5.73%)	57% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	93.16% <0%> (-5.13%)	2% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-2.84%)	31% <0%> (-2%)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
... and 23 more

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