Update example config to include mod_http_upload custom headers (#3288)

ejabberd.yml.example

@@ -162,6 +162,10 @@ modules:
   mod_http_api: {}
   mod_http_upload:
     put_url: https://@HOST@:5443/upload
+    custom_headers:
+      "Access-Control-Allow-Origin": "https://@HOST@"
+      "Access-Control-Allow-Methods": "GET,HEAD,PUT,OPTIONS"
+      "Access-Control-Allow-Headers": "Content-Type"
   mod_last: {}
   mod_mam:
     ## Mnesia is limited to 2GB, better to use an SQL backend