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.
Sorry I didn't see this before.  This will work for our needs, but to make it really safe I think you should:
1) Include the parameter key in the cache key.
2) Enforce an ordering for parameters.
3) URL-encode the parameter value.