doc: Adjust reference to spice-gtk man page
The man page spice-gtk ships is named "spice-client", not "spice-gtk"
Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
https://bugzilla.redhat.com/show_bug.cgi?id=1477966