Use safer |to_json

roles/matrix-bot-maubot/templates/config/config.yaml.j2

@@ -64,7 +64,7 @@ server:
     appservice_base_path: /_matrix/app/v1
     # The shared secret to sign API access tokens.
     # Set to "generate" to generate and save a new token at startup.
-    unshared_secret: {{ matrix_bot_maubot_secret }}
+    unshared_secret: {{ matrix_bot_maubot_secret|to_json }}
 
 # Known homeservers. This is required for the `mbc auth` command and also allows
 # more convenient access from the management UI. This is not required to create