githooks: Adjust hooks for the ports tree

• Capitalize the topic line: this way the example is consistent with the desired style.
• Update the description of MFH.
• Point the Pull Request field to the ports repo on GitHub.

Reviewed by: mat, bapt
Differential Revision: https://reviews.freebsd.org/D29861

(cherry picked from commit 37dfa4eeea6de2ae698ca8bb0023835c9f28f7b9)