Fix typo in mount path
Fix typo in mount path

```bash
mattbostock@production-mirrorer-1:~$ mount | grep mirror
/dev/mapper/mirror-data on /var/lib/govuk_mirror type ext4 (rw)
```

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet