mtg-config.toml: bind-to both IPv4/6

contrib/sni-router/mtg-config.toml

 # 3. Replace example.com with your actual domain everywhere.
 
 secret = "PASTE_YOUR_SECRET_HERE"
-bind-to = "0.0.0.0:3128"
+bind-to = "[::]:3128"
 
 # HAProxy in front sends PROXY protocol v2 headers so mtg can see the
 # real client IP.  Keep this in sync with haproxy.cfg (`send-proxy-v2`).