Fix StringIO import for the example in usage.rst

For some reason people fixed the `print` call but not the import when https://github.com/sqlalchemy/mako/issues/234 was brought up.

I also mentioned in in https://github.com/sqlalchemy/mako/issues/369

Closes: #370
Pull-request: https://github.com/sqlalchemy/mako/pull/370
Pull-request-sha: 8560bfed22f11b7edb5d2ac2e5cfc6a7bd59b718

Change-Id: I236eef05f5b45420035c522287fa66eb3cb21a00