dialog: clarify usage scope of $DLG_dir

Reported by @imdrpn in #2132

(cherry picked from commit 896be26)

modules/dialog/doc/dialog_admin.xml

@@ -3164,7 +3164,7 @@ event_route[E_RTPPROXY_DTMF] {
 			Returns the direction of the request in dialog (as "upstream" string
 			if the request is generated by callee or "downstream" string if the
 			request is generated by caller) - to be used for sequential request.
-			This PV will be available only for sequential requests (on
+			This PV will be available only for sequential requests (not for
 			replies), after doing loose_route().
 			</para>
 			<para>