bug#20203: add example to date man page

[積丹尼 Dan Jacobson]

Please change

       -I[TIMESPEC], --iso-8601[=TIMESPEC]
              output date/time in ISO 8601 format.  TIMESPEC='date'  for  date
              only  (the  default), 'hours', 'minutes', 'seconds', or 'ns' for
              date and time to the indicated precision.

to
        -I[TIMESPEC], --iso-8601[=TIMESPEC]
              output date/time in ISO 8601 format.  TIMESPEC='date'  for  date
              only  (2015-03-26, the  default), 'hours', 'minutes', 'seconds', 
or 'ns' for
              date and time to the indicated precision.