Event::sendEmail() - Fix displaying of heading. Use same key.

Event::sendEmail() - Fix displaying of heading. Use same key.

The message templates use this loop:

```
{foreach from=$customPost item=customPos key=j}
   <tr> <th {$headerStyle}>{$customPost_grouptitle.$j}</th></tr>
```

Note that the keys in `$customPost` and `$customPost_grouptitle` need to
match.