chore: Update blackend-docs repo

[henryiii]

Committed via https://github.com/asottile/all-repos

* Update blackend-docs pre-commit hook repo to
  https://github.com/adamchainz/blacken-docs.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0189237) 98.30% compared to head (709f1b6) 98.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2305   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          69       69           
  Lines        4536     4536           
  Branches      802      802           
=======================================
  Hits         4459     4459           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	97.88% <ø> (ø)
doctest	61.04% <ø> (ø)
unittests-3.10	96.31% <ø> (ø)
unittests-3.11	96.31% <ø> (ø)
unittests-3.8	96.34% <ø> (ø)
unittests-3.9	96.36% <ø> (ø)

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[matthewfeickert]

Thanks @henryiii!