Merge pull request #889 from dongweiming/doc

Fix blueprint doc

docs/sanic/blueprints.md

@@ -172,7 +172,7 @@ takes the format `<blueprint_name>.<handler_name>`. For example:
 ```python
 @blueprint_v1.route('/')
 async def root(request):
-    url = app.url_for('v1.post_handler', post_id=5) # --> '/v1/post/5'
+    url = request.app.url_for('v1.post_handler', post_id=5) # --> '/v1/post/5'
     return redirect(url)