Fixed typo in react-native browser instructions

[shilman]

Issue: N/A

What I did

Minor typo

How to test

Did not test. Followed instructions from README.md and also from https://facebook.github.io/react-native/docs/getting-started.html

[codecov]

Codecov Report

Merging #1189 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master    #1189    +/-   ##
========================================
  Coverage   13.31%   13.31%            
========================================
  Files         199      199            
  Lines        4589     4589            
  Branches      541      643   +102     
========================================
  Hits          611      611            
+ Misses       3492     3415    -77     
- Partials      486      563    +77

Impacted Files	Coverage Δ
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
.../ui/src/modules/ui/components/layout/dimensions.js	15.62% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.14% <0%> (ø)	⬆️
app/react/src/server/iframe.html.js	0% <0%> (ø)	⬆️
addons/info/src/components/Props.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/render.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.41% <0%> (ø)	⬆️
addons/knobs/src/KnobStore.js	6.81% <0%> (ø)	⬆️
app/react/src/client/preview/client_api.js	39.28% <0%> (ø)	⬆️
... and 24 more

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[tmeasday]

LGTM