Typo fix s/an hard-written string/a hard-written string/
Typo fix s/an hard-written string/a hard-written string/

As requested by @fzaninotto at https://github.com/marmelab/admin-on-rest/pull/77#discussion_r97550204
marmelab/admin-on-rest,matteolc/admin-on-rest,matteolc/admin-on-rest,marmelab/admin-on-rest