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

src/applications/policy/controller/PhabricatorPolicyExplainController.php

@@ -40,7 +40,7 @@ final class PhabricatorPolicyExplainController
 
     $dialog = id(new AphrontDialogView())
       ->setUser($viewer)
-      ->setClass('aphront-access-dialog')
+      ->setClass('aphront-access-dialog aphront-policy-explain-dialog')
       ->setTitle(pht('Policy Details: %s', $object_name))
       ->addCancelButton($object_uri, pht('Done'));