Fix nits typos and translation (#388)

internal/i18n/locales/en-US.yml

@@ -261,6 +261,7 @@ admin.invitations.code: Code
 admin.invitations.copy_link: Copy link
 admin.invitations.uses: Uses
 admin.invitations.expired: Expired
+admin.invitations.delete_confirm: Do you want to delete this invitation ?
 
 flash.admin.user-deleted: User has been deleted
 flash.admin.gist-deleted: Gist has been deleted