Update README.md

Author: depler

README.md

@@ -19,7 +19,7 @@ engine.AddFilterInV4(false);
 engine.AddFilterInV6(false);
 
 //allow incoming ping requests
-engine.AddFilterInV6(true, new[] { Native.IPPROTO.ICMP });
+engine.AddFilterInV4(true, new[] { Native.IPPROTO.ICMP });
 
 //allow incoming requests for DNS, HTTP, HTTPS
 engine.AddFilterInV4(true, new[] { Native.IPPROTO.TCP }, new [] { "53", "80", "443" });