Update copy used in the checkbox label

Updating the copy used in the email opt-in checkbox label as per [ECOM-687](https://openedx.atlassian.net/browse/ECOM-687). This copy is pending approval from @griffresch.

@dianakhuang, could you take a quick look? This just changes the value of a string, so I don't think two reviewers are needed. I'll merge once I get sign-off from Griff.
Does it make sense to mark this string or the one below for translation? Everything in the iframe is forced to English, and the sentence constructed by the courseware views may contain the English "and." Is it worth revisiting the string construction in the courseware views to make it more i18n friendly?