Updated en_US.yml translation file so SendEmailFromField title states "Reply To" not "From".  Aims to fix #81 (https://github.com/silverstripe/silverstripe-userforms/issues/81)

...f SendEmailFromField (Used for 'Reply To' not 'From')

Updated en_US.yml translation file to more accurately reflect function
of SendEmailFromField (Used for 'Reply To' not 'From').  en.yml seems
to be correct. Also updated REPLYADDRESS to be more consistent
(capitalisation).