dispatcher: set sock_avp parameter in example

modules/dispatcher/doc/dispatcher.cfg

@@ -115,6 +115,7 @@ modparam("dispatcher", "flags", 2)
 modparam("dispatcher", "dst_avp", "$avp(AVP_DST)")
 modparam("dispatcher", "grp_avp", "$avp(AVP_GRP)")
 modparam("dispatcher", "cnt_avp", "$avp(AVP_CNT)")
+modparam("dispatcher", "sock_avp", "$avp(AVP_SOCK)")
 
 ####### Routing Logic ########