Update Admin-API link

the old one is deprecated (says the page at the link target), and refers to this new one.

docs/configuring-playbook-synapse-admin.md

@@ -15,7 +15,7 @@ Add the following configuration to your `inventory/host_vars/matrix.DOMAIN/vars.
 matrix_synapse_admin_enabled: true
 ```
 
-**Note**: Synapse Admin requires Synapse's [Admin APIs](https://github.com/matrix-org/synapse/tree/master/docs/admin_api) to function. Access to them is restricted with a valid access token, so exposing them publicly should not be a real security concern. Still, for additional security, we normally leave them unexposed, following [official Synapse reverse-proxying recommendations](https://github.com/matrix-org/synapse/blob/master/docs/reverse_proxy.md#synapse-administration-endpoints). Because Synapse Admin needs these APIs to function, when installing Synapse Admin, we **automatically** exposes them publicly for you (equivalent to `matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_synapse_admin_api_enabled: true`).
+**Note**: Synapse Admin requires Synapse's [Admin APIs](https://matrix-org.github.io/synapse/latest/) to function. Access to them is restricted with a valid access token, so exposing them publicly should not be a real security concern. Still, for additional security, we normally leave them unexposed, following [official Synapse reverse-proxying recommendations](https://github.com/matrix-org/synapse/blob/master/docs/reverse_proxy.md#synapse-administration-endpoints). Because Synapse Admin needs these APIs to function, when installing Synapse Admin, we **automatically** exposes them publicly for you (equivalent to `matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_synapse_admin_api_enabled: true`).
 
 
 ## Installing