Fix the wrong link to nonexisted doc

Signed-off-by: Baohua Yang <yangbaohua@gmail.com>

README.md

@@ -107,7 +107,7 @@ If environment is prepared, then we can start cello service.
 
 ## Documentation, Getting Started and Develop Guideline
 
-For new users, it is highly recommended to read the [tutorial](docs/tutorial.md) or [index](docs/index.md) first.
+For new users, it is highly recommended to read the [documentation](docs/index.md) first.
 
 And feel free to visit the [online documentation](http://cello.readthedocs.io/en/latest/) for more information. You can also run `make doc` to start a local documentation website (Listen at [localhost:8000](http://127.0.0.1:8000).