Fix slash commands spec error

Should fix https://gitlab.com/gitlab-org/gitlab-ce/builds/9858509

Error:

1) Merge Requests > User uses slash commands merge-request-only commands /target_branch command in merge request changes target_branch in new merge_request
     Failure/Error: fill_in "merge_request_title", with: 'My brand new feature'

     Capybara::ElementNotFound:
       Unable to find field "merge_request_title"

My revision to this file makes sure that the tests click through to the next page.