Fix incorrect use of `$wpdb->prepare()`

Resolves this error:

```
Warning: Missing argument 2 for wpdb::prepare(), called in /srv/www/digitalfirstmedia.dev/content/mu-plugins/unconfirmed/unconfirmed.php on line 573 and defined in /srv/www/digitalfirstmedia.dev/wp/wp-includes/wp-db.php on line 1138
```
Merge pull request #15 from DFMdata/fix-incorrect-prepare

Fix incorrect use of `$wpdb->prepare()`