Fix typo in `setup:use-auth`

In the text displayed to the user by the `setup:use-auth` function in
`lib/git-hub.d/git-hub-setup`, the word “it” had been typo’d to “is”.

This commit fixes this typo.
Merge pull request #60 from 8573/8573/fix/text/setup-use-auth-typo/1

Fix typo in `setup:use-auth`