domiftune: Reword bandwidth clearing paragraph

s/of value/value of/
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tools/virsh.pod

@@ -699,7 +699,7 @@ in a single burst at -I<peak> speed as described in the Network XML
 documentation at L<http://libvirt.org/formatnetwork.html#elementQoS>.
 
 To clear inbound or outbound settings, use I<--inbound> or I<--outbound>
-respectfully with average of value zero.
+respectfully with average value of zero.
 
 If I<--live> is specified, affect a running guest.
 If I<--config> is specified, affect the next boot of a persistent guest.