Fix typo in contributing guide

[slorber]

Issue:

What I did

How to test

[shilman]

LGTM! Thanks! 👍

[codecov]

Codecov Report

Merging #1273 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1273   +/-   ##
=======================================
  Coverage   13.73%   13.73%           
=======================================
  Files         207      207           
  Lines        4638     4638           
  Branches      519      519           
=======================================
  Hits          637      637           
- Misses       3551     3552    +1     
+ Partials      450      449    -1

Impacted Files	Coverage Δ
.../ui/src/modules/ui/components/left_panel/header.js	27.58% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	0% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
app/react/src/client/preview/init.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/error_display.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.86% <0%> (ø)	⬆️
...src/server/config/WatchMissingNodeModulesPlugin.js	0% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	0% <0%> (ø)	⬆️
... and 11 more

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