Update _form.html.haml

Open Jennifer Garcia requested to merge jenibella/gitlab-ee:patch-3 into master

What does this MR do?

Makes it more clear how to link a URL.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Unclear instruction, not everyone knows what page slug is.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?