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.
Must do this:
```python
ungettext(
    "I would like to receive email from {organization_list} and learn about its other programs.",
    "I would like to receive email from {organization_list} and learn about their other programs.",
    len(org_list)
).format(organization_list=org_name_string)
```

ALWAYS you must FIRST get the result from u(n)gettext (perform the call to the gettext function) AND THEN use .format on the _result that the gettext call returns_.