Fix format for date field on example config.json

[dinoshauer]

Running tap-salesforce --config config.json --discover with the date format from the example config raises an error, it's a pretty straight forward formatting error, still it'd be nice if the correct format was in the README :)

For good measure, this is the output of the above command

$ tap-salesforce --config config.json --discover
CRITICAL time data '2017-11-02' does not match format '%Y-%m-%dT%H:%M:%SZ'
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/singer/utils.py", line 28, in strptime
    return datetime.datetime.strptime(dtime, DATETIME_FMT)
  File "/usr/local/lib/python3.7/_strptime.py", line 577, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
  File "/usr/local/lib/python3.7/_strptime.py", line 351, in _strptime
    (bad_directive, format)) from None
ValueError: '0' is a bad directive in format '%04Y-%m-%dT%H:%M:%S.%fZ'

Edit: Also fixes the example command to run in discover mode

[cmerrick]

Hi @dinoshauer, thanks for your contribution!

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[cmerrick]

You did it @dinoshauer!

Thank you for signing the Singer Contribution License Agreement.

[briansloane]

Thanks @dinoshauer! This looks great.