add an necessary import for better understanding

add `from sanic.response import redirect`

docs/sanic/routing.md

@@ -138,6 +138,8 @@ app.add_route(person_handler2, '/person/<name:[A-z]>', methods=['GET'])
 Sanic provides a `url_for` method, to generate URLs based on the handler method name. This is useful if you want to avoid hardcoding url paths into your app; instead, you can just reference the handler name. For example:
 
 ```python
+from sanic.response import redirect
+
 @app.route('/')
 async def index(request):
     # generate a URL for the endpoint `post_handler`
@@ -145,7 +147,6 @@ async def index(request):
     # the URL is `/posts/5`, redirect to it
     return redirect(url)    
 
-
 @app.route('/posts/<post_id>')
 async def post_handler(request, post_id):
     return text('Post - {}'.format(post_id))