[Committers Guide] Fix paths in Steps for New Committers

[fernape]

Summary

There is no autors.toml

freebsd-doc$ find . -name authors.toml
freebsd-doc$ find . -name authors.adoc
./shared/authors.adoc

Also add doc in the path as in the other steps to make explicit that we refer
to the doc repository.

s/shared/share in src repository:

freebsd-src$ ls shared/misc/committers*.dot
ls: shared/misc/committers*.dot: No such file or directory
freebsd-src$ ls share/misc/committers*.dot
share/misc/committers-doc.dot    share/misc/committers-ports.dot share/misc/committers-src.dot

Test Plan

• Repo builds fine
• Committers Guide renders properly

[0mp]

LGTM!