Fix error message when invitation has expired

The error message for when an invitation to Notify had expired was
displaying in admin with square brackets round it because admin is not
expecting the message to be a list
(a85134ee22/app/models/user.py (L500))

app/accept_invite/rest.py

@@ -35,8 +35,8 @@ def validate_invitation_token(invitation_type, token):
                                       max_age_seconds)
     except SignatureExpired:
         errors = {'invitation':
-                  ['Your invitation to GOV.UK Notify has expired. '
-                   'Please ask the person that invited you to send you another one']}
+                  'Your invitation to GOV.UK Notify has expired. '
+                  'Please ask the person that invited you to send you another one'}
         raise InvalidRequest(errors, status_code=400)
     except BadData:
         errors = {'invitation': 'Something’s wrong with this link. Make sure you’ve copied the whole thing.'}