Update Caddy example in configuring-well-known to Caddy version 2

docs/configuring-well-known.md

@@ -107,7 +107,9 @@ server {
 **For Caddy**, it would be something like this:
 
 ```caddy
-proxy /.well-known/matrix https://matrix.DOMAIN
+reverse_proxy /.well-known/matrix/* https://matrix.DOMAIN {
+	header_up Host {http.reverse_proxy.upstream.hostport}
+}
 ```
 
 **For HAProxy**, it would be something like this: