freedombox-manual.xml does not validate

I skipped fetching the manual for v0.9.2, but we will need to correct these errors for the next release.

~/fbx/plinth/doc$ make fetch && make
wget --quiet -O - "https://wiki.debian.org/FreedomBox/Manual?action=show&mimetype=text%2Fdocbook" | \
  xmllint --format --output freedombox-manual.raw.xml -
xsltproc --output freedombox-manual.xml fixes.xslt freedombox-manual.raw.xml
xsltproc fetch-images.xslt freedombox-manual.raw.xml | sort -u | \
  awk '{print "wget --quiet -O images/" $1 " " $2}' | sh
rm -f freedombox-manual.raw.xml
xmlto --with-dblatex pdf freedombox-manual.xml
xmlto: /home/jvalleroy/fbx/plinth/doc/freedombox-manual.xml does not validate (status 3)
xmlto: Fix document syntax or use --skip-validation option
/home/jvalleroy/fbx/plinth/doc/freedombox-manual.xml:3459: element link: validity error : Syntax of value for attribute linkend of link is not valid
/home/jvalleroy/fbx/plinth/doc/freedombox-manual.xml:3487: element link: validity error : Syntax of value for attribute linkend of link is not valid
/home/jvalleroy/fbx/plinth/doc/freedombox-manual.xml:3487: element link: validity error : IDREF attribute linkend references an unknown ID "Finding out the IP address of the virtual machine"
/home/jvalleroy/fbx/plinth/doc/freedombox-manual.xml:3459: element link: validity error : IDREF attribute linkend references an unknown ID "Network Configuration"
Document /home/jvalleroy/fbx/plinth/doc/freedombox-manual.xml does not validate
Makefile:50: recipe for target 'freedombox-manual.pdf' failed
make: *** [freedombox-manual.pdf] Error 13