books/porters-handbook: Fix numerous mismatches in section 11.1

In step 1 users were told to do "cd ~/my_ports_wrkdir" but after clone
the repository users were instead told to do "cd ~/my_wrkdir". Also, the
git command in step 2 is not followed by either of them. The mirror
sites table is outdated as well.

This fix changes the whole shell code block to use the directory name
my_wrkdir. Also, the repository URL table xref replaces the old SVN
mirror sites xref.

Reported by: Pau Amma <pauamma@gundo.com>
Approved by: 0mp
Differential Revision: https://reviews.freebsd.org/D29916