check_base_has_width: use correct param

[m4rc1e]

Description

CI is failing for Merriweather, google/fonts#7886 (comment):

ERROR Check base characters have non-zero advance width. [base_has_width](https://fontbakery.readthedocs.io/en/stable/fontbakery/checks/universal.html#base-has-width)
💥 ERROR
Failed with AttributeError: 'CheckRunContext' object has no attribute 'get'

The check is currently trying to pass a context variable into a bullet list, when it actually needs a config variable.

Checklist

• update CHANGELOG.md
• wait for the tests to pass
• request a review