Fix the refered link

Prior to this change, the refered wiki link in acl.conf has been
deprecated.

This change replace the deprecated link with the doc link in official site.

etc/acl.conf

@@ -1,6 +1,6 @@
 %%--------------------------------------------------------------------
 %%
-%% [ACL](https://github.com/emqtt/emqttd/wiki/ACL)
+%% [ACL](http://emqtt.io/docs/v2/config.html#allow-anonymous-and-acl-file)
 %%
 %% -type who() :: all | binary() |
 %%                {ipaddr, esockd_access:cidr()} |