Help messages don't support formatting

[cfournie]

This PR rephrases the help messages to remove formatting elements. This changes:

$ pylint --help-msg C2601
 :import-modules-only (C2601): *%(child)s is not a module or cannot be imported*
  Install %(child)s or only import packages or modules from it This message
  belongs to the google-styleguide-checker checker.

To now read:

$ pylint --help-msg C2601
 :import-modules-only (C2601): *%(child)s is not a module or cannot be imported*
  Only import packages or modules and ensure that they are installed. This message
  belongs to the google-styleguide-checker checker.

[honkfestival]

LGTM