handbook/advanced-networking: fix internal xrefs

[concussious.bugzilla_runbox.com]

Summary

After testing a previous revision, I discovered internal xrefs generated with crossref: do not work at all on PDF [0].
This revision updates all of the internal links in this chapter to use the syntax instructed in Asciidoctor official doc on xrefs [1].

I am currently going through the rest of the handbook for these while digesting the considerations from x11 refactor, this seems like a great task for me.

[0] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266107
[1] https://docs.asciidoctor.org/asciidoc/latest/macros/xref/

Test Plan

Links worked in PDF located at doc/documentation/public/en/books/handbook/handbook_en.pdf generated with make pdf-books DOC_LANG="en" HUGO_ARGS="--verbose --debug".

[concussious.bugzilla_runbox.com]

Forgot to capitalize "Network Chapter", as it was previously.

[fernape]

See https://reviews.freebsd.org/D46266#inline-277720 before working on this

[concussious.bugzilla_runbox.com]

Closing: overcome by events: Fernape fixed the internal macro in D46480!