Fix the cross-link from register admin API to config docs. (#13870)

changelog.d/13870.doc

+Fix a cross-link from the register admin API to the `registration_shared_secret` configuration documentation.

docs/admin_api/register_api.md

@@ -5,7 +5,7 @@ non-interactive way. This is generally used for bootstrapping a Synapse
 instance with administrator accounts.
 
 To authenticate yourself to the server, you will need both the shared secret
-([`registration_shared_secret`](../configuration/config_documentation.md#registration_shared_secret)
+([`registration_shared_secret`](../usage/configuration/config_documentation.md#registration_shared_secret)
 in the homeserver configuration), and a one-time nonce. If the registration
 shared secret is not configured, this API is not enabled.