Fix caddy .well-known config

I assume you'll also need to change this for nginx and apache. Without this, Riot will tell you "Cannot find homeserver".

docs/configuring-well-known.md

@@ -71,6 +71,7 @@ server {
 
 ```caddy
 proxy /.well-known/matrix https://matrix.DOMAIN
+header /.well-known/matrix Access-Control-Allow-Origin  *
 ```
 
 Make sure to: