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.
This is really horrific for i18n.  I know we're forcing the iframe to English anway (for now at least), but there has to be a better way to handle this.  @sarina is there an i18n best practice for comma-separated lists with "and" before the last item?