fix typo (Response -> Request)

check docs for more information b5c552d17f/docs/topics/spider-middleware.rst

scrapy/templates/project/module/middlewares.py.tmpl

@@ -39,7 +39,7 @@ class ${ProjectName}SpiderMiddleware(object):
         # Called when a spider or process_spider_input() method
         # (from other spider middleware) raises an exception.
 
-        # Should return either None or an iterable of Response, dict
+        # Should return either None or an iterable of Request, dict
         # or Item objects.
         pass