docs(async): Fix pluralization in docstring

[Zearin]

I was just reading the docs on https://jsr.io/@std/async. The links are plural, but also formatted inside <code> elements.

Not a big deal, but it’s nicer when the link to docs for a function/module/class/etc. exactly matches the name of the linked target.

With the addition of a single comma, the pluralized links can be restored to their singular counterparts, and the sentence still makes sense.

[CLAassistant]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.29%. Comparing base (6b93b78) to head (2c65e4c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6946   +/-   ##
=======================================
  Coverage   94.28%   94.29%           
=======================================
  Files         584      584           
  Lines       43186    43186           
  Branches     6933     6933           
=======================================
+ Hits        40720    40723    +3     
+ Misses       2413     2411    -2     
+ Partials       53       52    -1     

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