Minorly correct `config-not-setup` text

Apply the following minor corrections to the text printed by the
`config-not-setup` function in `lib/git-hub`:

- De-duplicate a duplicate “a”.
- Change “setup”, used as an adjective, to “set up” (“setup” is a
noun).
- Change a “which” at the start of a sentence to “This”.
- Add a missing “to” after a “Would you like”.