fix readme

[drmorr0]

The README seems to reference a from_str function that doesn't exist any more. The following code doesn't work for me:

parse_datetime::parse_datetime::from_str(tstr)

returning

failed to resolve: expected type, found function `parse_datetime` in `parse_datetime`

If instead I change it to

parse_datetime::parse_datetime(tstr)

it works fine.

[tertsdiepraam]

Thanks!