Update branch name in install instructions.

[hexacera]

╰─❯ pip install git+https://github.com/geigerzaehler/beets-check.git@master

Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple
Collecting git+https://github.com/geigerzaehler/beets-check.git@master
  Cloning https://github.com/geigerzaehler/beets-check.git (to revision master) to /tmp/pip-req-build-wwrf5_ww
  Running command git clone --filter=blob:none --quiet https://github.com/geigerzaehler/beets-check.git /tmp/pip-req-build-wwrf5_ww
  WARNING: Did not find branch or tag 'master', assuming revision or ref.
  Running command git checkout -q master
  error: pathspec 'master' did not match any file(s) known to git
  error: subprocess-exited-with-error

  × git checkout -q master did not run successfully.
  │ exit code: 1
  ╰─> See above for output.

[geigerzaehler]

Thanks for fixing this!