collaborator_guide: clarify commit message rules

Pull Request check-list

Please make sure to review and check all of these items:

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with this change (including linting)?
• Is the commit message formatted according to [CONTRIBUTING.md][0]?
• If this change fixes a bug (or a performance problem), is a regression test (or a benchmark) included?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

Affected core subsystem(s)

None, just the contributing_guide

Description of change

Italicize the full URL being required in metadata. I have added commits to core that lacked the full URL, as have others in the past. I want to make it more obvious for future contributors.