Miscellaneous small changes to the Committer's Guide.

PR below plus a few more I spotted while working on it.

While there, check nearby URLs still work and use https:.

PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261812
Reviewed by: carlavlla, gjb, philip
Approved by: carlavilla (mentor), gjb (mentor, earlier version)
Differential Revision: https://reviews.freebsd.org/D37091