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
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