Fix DatetimeNow macro example

[sowla]

as it currently refers to a non-existent transform and parameter.

Description of changes:

1. change the transform name from 'String' to 'DatetimeNow' When I run the example as is, I get the error No transform named <account ID>::String found.

2. change the parameter name from 'param' to 'format' When I run the example as is, I get the error Transform <account ID>::DatetimeNow failed with: 'format'

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.