documentation: Update proxy docs to point to WithContextDialer (#7361)

Documentation/proxy.md

@@ -12,4 +12,4 @@ connection before giving it to gRPC.
 
 If the default proxy doesn't work for you, replace the default dialer with your
 custom proxy dialer. This can be done using
-[`WithDialer`](https://godoc.org/google.golang.org/grpc#WithDialer).
+[`WithContextDialer`](https://pkg.go.dev/google.golang.org/grpc#WithContextDialer).