Change sample config's postgres user to synapse_user (#7889)

The [postgres setup docs](https://github.com/matrix-org/synapse/blob/develop/docs/postgres.md#set-up-database) recommend setting up your database with user `synapse_user`.

However, uncommenting the postgres defaults in the sample config leave you with user `synapse`.

This PR switches the sample config to recommend `synapse_user`. Took a me a second to figure this out, so assume this will beneficial to others.