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.
Hmm, on second thought, encoding this as ASCII should be sufficient (assuming that `safe_key` is working correctly).  So maybe just do:
```
cache_key += '.' + unicode(parameter_value).encode('utf-8')
```