doc: Remove mention of FreeBSD using DocBook

The FreeBSD Project has switched documentation markup format from
DocBook to Asciidoc, so remove the reference on our documentation
page.

Reported-by: Matemática A3K <matematica.a3k@gmail.com>
Discussion: https://postgr.es/m/CA+FDnhJFvFGk7Kdc=yX9=3aeRC3p4bdfYLX+x5GbOcGNKtWwkw@mail.gmail.com

doc/src/sgml/docguide.sgml

@@ -61,9 +61,6 @@
    OASIS group</ulink>.  The <ulink url="https://www.oasis-open.org/docbook/">
    official DocBook site</ulink> has good introductory and reference documentation and
    a complete O'Reilly book for your online reading pleasure.
-   The <ulink url="https://www.freebsd.org/docproj/">
-   FreeBSD Documentation Project</ulink> also uses DocBook and has some good
-   information, including a number of style guidelines that might be
    worth considering.
   </para>
  </sect1>