docs: remove transport url config

[maxhelias]

Fix a small oversight #766

[7-zete-7]

There is no need to remove the configuration of this option. I was hasty in defining the default value for the Mercure path option 😅

Mercure will store its data at ./bolt.db (/app/bolt.db) if the environment variable MERCURE_TRANSPORT_URL is not set. In #768, the use of this environment variable should return.