Copied the missing key from the core

This key is defined like this in the core, but it's missing from the spree_i18n. This is my attempt to make the string translatable in the Localeapp.

https://github.com/spree/spree/blob/master/core/config/locales/en.yml#L777
Merge pull request #341 from vkvelho/master

Copied the missing key from the core