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

hgext/blackbox.py

@@ -38,7 +38,7 b' Examples::'
   [blackbox]
   # Include nanoseconds in log entries with %f (see Python function
   # datetime.datetime.strftime)
-  date-format = '%Y-%m-%d @ %H:%M:%S.%f'
+  date-format = %Y-%m-%d @ %H:%M:%S.%f
 
 """