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