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

[khng]

Summary

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sites table is outdated as well.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 <[e-mail]>

[0mp]

documentation/content/en/books/porters-handbook/upgrading/_index.adoc

90

Id' remove this line completely.

91

[khng]

Remove the first cd completely, and prepend the git target directory path with tilde

[0mp]

documentation/content/en/books/porters-handbook/upgrading/_index.adoc

96

As we are fixing this example anyway, perhaps we could point to the correct section in the handbook as well. For example, to the git-url-table in the mirrors chapter?

[0mp]

documentation/content/en/books/porters-handbook/upgrading/_index.adoc

96

As of 16ca20a527bce7d537375f6fab179549c07fe723 it is called "FreeBSD Git Repository URL Table".

[khng]

Fix the repo xref as well.

[0mp]

Awesome, thanks a lot!