Fix file name in viewport addon usage instructions

[klaussner]

What I did

Changed addon.js to addons.js. 🙂

[codecov]

Codecov Report

Merging #2563 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2563   +/-   ##
=======================================
  Coverage   32.68%   32.68%           
=======================================
  Files         398      398           
  Lines        8838     8838           
  Branches      932      955   +23     
=======================================
  Hits         2889     2889           
+ Misses       5332     5275   -57     
- Partials      617      674   +57

Impacted Files	Coverage Δ
app/react/src/server/utils.js	0% <0%> (-53.58%)	⬇️
lib/ui/src/modules/ui/containers/search_box.js	23.52% <0%> (ø)	⬆️
...modules/ui/components/stories_panel/text_filter.js	30.98% <0%> (ø)	⬆️
...res__/update-addon-info/update-addon-info.input.js	0% <0%> (ø)	⬆️
addons/graphql/src/components/FullScreen/index.js	0% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-build.js	0% <0%> (ø)	⬆️
...-native/src/preview/components/OnDeviceUI/style.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/stories_panel.js	25.71% <0%> (ø)	⬆️
addons/actions/src/lib/util/typeReplacer.js	54.54% <0%> (ø)	⬆️
... and 40 more

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