Remove incorrect preview- prefix

[rthor]

The docs suggested an incorrect filename for custom head tags.

[codecov]

Codecov Report

Merging #1375 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1375   +/-   ##
=======================================
  Coverage   14.35%   14.35%           
=======================================
  Files         201      201           
  Lines        4613     4613           
  Branches      506      504    -2     
=======================================
  Hits          662      662           
- Misses       3508     3521   +13     
+ Partials      443      430   -13

Impacted Files	Coverage Δ
lib/ui/src/modules/shortcuts/actions/shortcuts.js	6.25% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
addons/info/src/components/PropTable.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Color.js	8% <0%> (ø)	⬆️
addons/info/src/components/PropVal.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
addons/knobs/src/components/types/Object.js	5.81% <0%> (ø)	⬆️
app/react/src/client/preview/client_api.js	39.28% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.93% <0%> (ø)	⬆️
... and 15 more

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[shilman]

@rthor preview-head.html is correct. head.html is also supported for backwards, compatibility, but is deprecated. it was renamed because we also added manager-head.html to add custom head tags for the whole storybook window instead of just the preview iframe.

If you're not seeing that perhaps you're using an older version of storybook?

[shilman]

@rthor closing this. please correct me if I got something wrong!