blackbox: Remove misleading quotes in config example This example previously looked like quotes were part of configuration file syntax, and the parsed `date-format` value was the part inside of them. This is not the case: config syntax only parses quotes in list values. Instead using that config would result in literal quotes being written to `.hg/blackbox.log` as part of the date format. This changes the example to what was probably intended. Differential Revision: https://phab.mercurial-scm.org/D10011