fix: regex to catch inner functions doesn't catch asynchronous ones

[matthieucan]

Hello, and thanks for the great tool!

This small PR fixes the case of detecting asynchronous inner functions, for D202 (inner classes and functions require an empty line between the outer function docstring and them). It is a follow-up of #426.

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

[sambhav]

LGTM, thanks for the contribution!