docs: provide async for request body

[iiio2]

Reference: #
There is await readBody inside defineEventHandler. It needs an async function.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.40%. Comparing base (a58d7c9) to head (de41033).
Report is 78 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #777      +/-   ##
==========================================
+ Coverage   77.83%   79.40%   +1.57%     
==========================================
  Files          47       54       +7     
  Lines        4286     5327    +1041     
  Branches      611      675      +64     
==========================================
+ Hits         3336     4230     +894     
- Misses        933     1078     +145     
- Partials       17       19       +2     

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