[Brent] Improve message in referral email.

[dracos]

For https://mysocietysupport.freshdesk.com/a/tickets/4308 (Also the last commit on master above this wasn't properly reviewed, as urgent bug fix, but if you had a quick look at that too, thanks.) [skip changelog]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.66%. Comparing base (9e18f67) to head (abf6e27).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5041 +/- ## ========================================== - Coverage 82.66% 82.66% -0.01% ========================================== Files 396 396 Lines 30819 30821 +2 Branches 4889 4889 ========================================== + Hits 25478 25479 +1 - Misses 3896 3897 +1 Partials 1445 1445 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.