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.
@wedaly, these strings aren't currently marked for translation, as per the comment. Are you suggesting marking them as such? I spoke with Diana about this; since these strings are not used across the system and we only use the English strings, translating these strings seems like it would be wasted effort on the part of our translators.