update default hookshot webhook mxid prefix

roles/matrix-bridge-hookshot/defaults/main.yml

@@ -86,7 +86,8 @@ matrix_hookshot_generic_enabled: true
 matrix_hookshot_generic_endpoint: "{{ matrix_hookshot_webhook_endpoint }}"
 matrix_hookshot_generic_urlprefix: "{{ matrix_server_fqn_matrix }}{{ matrix_hookshot_generic_endpoint }}"
 matrix_hookshot_generic_allow_js_transformation_functions: false
-matrix_hookshot_generic_user_id_prefix: 'webhooks_'
+# if you're also using matrix-appservice-webhooks, take care that these prefixes don't overlap
+matrix_hookshot_generic_user_id_prefix: '_webhooks_'
 
 
 matrix_hookshot_figma_enabled: false