Reduce space under main heading of letter
Reduce space under main heading of letter

Measured difference is that the first line of body text on the DVLA
outputted PDF is 2.9mm higher up.

```
  2.9mm
--------
= 0.114"
--------
= 8.22pt

  8.35pt
- 8.22pt
--------
= 0.13pt
```

alphagov/notifications-utils