docs: fix `xl list' manpage entry

as it was not covering the '-n' option, which is present
since d743a223 ("xl: add node-affinity to the output of
`xl list`").

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

docs/man/xl.pod.1

@@ -304,6 +304,10 @@ Also prints the security labels.
 
 Also prints the domain UUIDs, the shutdown reason and security labels.
 
+=item B<-n>, <--numa>
+
+Also prints the domain NUMA node affinity.
+
 =back
 
 B<EXAMPLE>