Update config.md (#22115)

fix typo

Author: Moelf

docs/config.md

@@ -14,7 +14,7 @@ for the desktop app the application will need to be exited fully (including via
 ## Homeserver configuration
 
 In order for Element to even start you will need to tell it what homeserver to connect to *by default*. Users will be
-able to use a different homeserver if they like, though this can be disabled with `"disable_custom_urls": false` in your
+able to use a different homeserver if they like, though this can be disabled with `"disable_custom_urls": true` in your
 config.
 
 One of the following options **must** be supplied: