Followup / fixup: apply 'black' code formatting to a couple of missed docstrings

Followup / fixup: apply 'black' code formatting to a couple of missed docstrings

### Description
Follow-up / completion of #1220.  That change updated a number of docstrings within the codebase to use standardised `black` code formatting, but a couple of locations had been missed.

### Checklist

This pull request is:

- [x] A documentation / typographical error fix
	- Good to go, no issue or tests are needed

Closes: #1228
Pull-request: https://github.com/sqlalchemy/alembic/pull/1228
Pull-request-sha: f5696b9ca34998e22af5705434ed2393d86e82c0

Change-Id: I5d935b036d6f4e11eb5c229f9982db587d67ae24