small typo in README

[CPASimUSante]

This is not a big deal, but in the usage part, instead of
use Mablae\DatetimepickerBundle\Form\Type\DateTimeType;
this should be
use Mablae\DatetimepickerBundle\Form\Type\DatetimeType;

I didn't fork for this

Thanks for this useful bundle, and for the SF 3 integration :)