Translate a missing string. props ldebrouwer, fixes #16861.

git-svn-id: https://develop.svn.wordpress.org/trunk@18249 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/user-new.php

@@ -31,7 +31,7 @@ Please click the following link to activate your user account:
 	add_filter( 'wpmu_signup_user_notification_email', 'admin_created_user_email' );
 
 	function admin_created_user_subject( $text ) {
-		return "[" . get_bloginfo('name') . "] Your site invite";
+		return sprintf( __( '[ %s ] Your site invite' ), get_bloginfo( 'name' ) );
 	}
 }