Add missing colon on adjustment line of admin order form

I noticed the adjustment line of the admin order form is missing a trailing `:` which is present for order total and subtotal.

![missing-colon](https://f.cloud.github.com/assets/100218/92665/8ae38534-65e8-11e2-8437-1086d0128720.png)

The issue only affects the form. The admin order details view is fine.