Fix some policy CSS

Summary:
Ref T11853. My CSS change for the more enormous policy dialog was a little too broad, and affected the "You shall not pass!" dialog too.

Narrow the scope of the CSS rules.

Also add a missing "." that I caught.

Test Plan:

• Looked at policy exception dialogs.
• Looked at policy explanation dialogs.
• Looked at the end of that sentence.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11853

Differential Revision: https://secure.phabricator.com/D16841