Fixed wrong status code type

- `statusCode` should be a number, but we were passing a string
- this doesn't really affect anything, but tsserver was flagging it up
  as the wrong type

ghost/core/core/server/web/api/endpoints/admin/middleware.js

@@ -64,7 +64,7 @@ const notImplemented = function notImplemented(req, res, next) {
     next(new errors.InternalServerError({
         errorType: 'NotImplementedError',
         message: tpl(messages.notImplemented),
-        statusCode: '501'
+        statusCode: 501
     }));
 };