[committers-guide] Update src branches information

Summary:
s/SVN/Git
s/head/main

Although there are other branches in the repo (like vendor/ or releng/), if they
were not considered noteworthy before, I choose not make them notheworthy now.

Test Plan: * Renders OK.

Reviewers: 0mp, gbe

Differential Revision: https://reviews.freebsd.org/D29886