Refactor Edit on GitLab code

Small fix to: !1860 (merged).

We want to do a comparison, not an assignment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/2b939c475ea87fe110651808d896b1a15c4769a5/lib/helpers/edit_on_gitlab.rb#L26.

Causes the following when building the site:

Loading site… lib/helpers/edit_on_gitlab.rb:26: warning: found `= literal' in conditional, should be ==

While here, switches "if-else" style to case statements.

CC @axil @cnorris