docs: fix channel mention formatting

[mwbrooks]

Summary

This pull request closes #446 by updating the docs to use <!channel> instead of <@channel> when mentioning the channel. See special formatting documentation.

Big thanks to @jacklu97 for reporting this issue! 🙇🏻

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (f234e85) to head (40029e9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #447   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files           7        7           
  Lines         709      709           
=======================================
  Hits          708      708           
  Misses          1        1           

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[mwbrooks]

note: Editor inserted a newline at EOF