Fix addkeys.sh README link.
Current link in Committer's Guide for the README of the addkeys.sh script is
broken.
It can be seen here:
https://docs.freebsd.org/en/articles/committers-guide/#commit-steps
It points to http://svnweb.freebsd.org/doc/head/shared/pgpkeys/README which
returns 404. It should point to https://docs.freebsd.org/pgpkeys/README.