Fix a URL typo in the MR API specs

What does this MR do?

This fixes a small typo in a MR API URL reported at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15088/diffs#note_128560114.

The spec was passing (returning a 404 response) but not for the expected reason (MR couldn't be found).

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15088/diffs#note_128560114.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer