user-manual: run xsltproc without --nonet option

user-manual: run xsltproc without --nonet option

The --nonet option prevents xsltproc from going to the network to find
anything.  But it always tries to find them locally first, so for a
user with the necessary docbook stylesheets installed the build will
work just fine without xsltproc attempting to use the network; all
--nonet does is make it fail rather than falling back on that.  That
doesn't seem particularly helpful.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>