blackbox: correct date format doc

Differential Revision: https://phab.mercurial-scm.org/D12004

hgext/blackbox.py

@@ -36,7 +36,7 b' Examples::'
   maxfiles = 3
 
   [blackbox]
-  # Include nanoseconds in log entries with %f (see Python function
+  # Include microseconds in log entries with %f (see Python function
   # datetime.datetime.strftime)
   date-format = %Y-%m-%d @ %H:%M:%S.%f