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”.
Merge pull request #59 from 8573/8573/fix/text/config-not-setup-misc/1

Minorly correct `config-not-setup` text