lowercase head for contributing docs

[jkup]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Fix #5122

@bbondy

Test Plan:

Small doc fix to lowercase head while fetching a PR branch.

[bsclifton]

++