Replace references to svnadmin branch

[fernape]

Summary

In committers-guide, handbook and the new-account page in the website.
There were references to the old svnadmin branch.

tcberner@ told me exactly where to look for the new mentors file.

Test Plan

• make from doc repo top level directory
• Generated documents look OK
• Note that the generated links point to docs.freebsd.org not to our local repo and since the "admin-branch" anchor is not already there, it seems it will fail. It can be locally tested by using file:path_to_local_doc_repo/documentation/public/en/books/handbook/mirrors/index.html#admin-branch and checking that the anchor actually exists.

[0mp]

Looks good to me!

documentation/content/en/articles/committers-guide/_index.adoc

2434

[fernape]

Thanks for the review!