Fixed CLI output when running post_write hooks

Fixed CLI output when running post_write hooks

Fixes: #1261
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### Description
Just fixed badly written Python string interpolation. Used the `%` operator just like the lines above.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #1262
Pull-request: https://github.com/sqlalchemy/alembic/pull/1262
Pull-request-sha: 19d17f5d061319bbf142b45a593081da5fc1dda8

Change-Id: I89ef873935d7206a61f78a09139cd5ab192a2275