構文仕様のドキュメントを修正

[marihachi]

What

• メンション (resolve メンションのユーザー名に.が許可されたが、ドキュメントが更新されていない #166)

Why

Additional info (optional)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.59%. Comparing base (2648486) to head (e110337).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #167   +/-   ##
========================================
  Coverage    97.59%   97.59%           
========================================
  Files            7        7           
  Lines         1083     1083           
  Branches       304      304           
========================================
  Hits          1057     1057           
  Misses          26       26           

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[KisaragiEffective]

LGTM

[syuilo]

🙏🏻