fix first sentence in custom style docs

[hubwoop]

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Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
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• I've updated documentation as required by this PR.
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[codecov]

Codecov Report

Merging #10322 (5b59c4b) into master (6756a5f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10322   +/-   ##
=======================================
  Coverage   46.13%   46.13%           
=======================================
  Files         228      228           
  Lines       27961    27961           
=======================================
  Hits        12900    12900           
  Misses      13315    13315           
  Partials     1746     1746           

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[crenshaw-dev]

Thanks @hubwoop!