route: remove reference to long-gone netiso protocol in route(8).

sbin/route/route.8

@@ -354,7 +354,7 @@ The optional modifiers
 and
 .Fl ssthresh
 provide initial values to quantities maintained in the routing entry
-by transport level protocols, such as TCP or TP4.
+by transport level protocols, such as TCP.
 These may be individually locked by preceding each such modifier to
 be locked by
 the