[Docs] Fix CSP example
What does this MR do?
- adds required
frame_ancestor
(default isnil
) - fixes typo in
img_src
Related issues
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31402#note_206881153
frame_ancestor
(default is nil
)img_src
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31402#note_206881153