xml2nroff: Fix the support for h4.

Half of the man page generated from ovn-architecture.7.xml is truncated
after the line:

<h4> Physical VLAN MTU Issues</h4>

It is because of the synctax problem of the definition of .SU in xml2nroff,
which is used by h4.

Fixes: 852316e ("xml2nroff: Properly support <h4>.")
Acked-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Han Zhou <hzhou@ovn.org>

build-aux/xml2nroff

@@ -92,7 +92,7 @@ def manpage_to_nroff(xml_file, subst, include_path, version=None):
 ..
 .de SU
 .  PP
-.  I "\\\\$1"
+.  I "\\$1"
 ..
 build/''' % (build.nroff.text_to_nroff(program),
              build.nroff.text_to_nroff(section),