Add missing "omniauth" prefix to option in docs

Jonrequested to merge
joonty/gitlab-ce:omniauth-doc-fix into master

This MR is specifically a fix for #3319 (closed). I wanted to make my first contribution to GitLab, so I picked a simple one!

This changes the omniauth documentation for the block_auto_created_users option to omniauth_block_auto_created_users.

Thanks

closes #3319 (closed)

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