Fix tutorial instruction to also add pyyaml (#6443)

Author: briwa

docs/tutorial/7-schemas-and-client-libraries.md

@@ -29,9 +29,10 @@ automatically generated schemas. Since we're using viewsets and routers,
 we can simply use the automatic schema generation.
 
 You'll need to install the `coreapi` python package in order to include an
-API schema.
+API schema, and `pyyaml` to render the schema into the commonly used
+YAML-based OpenAPI format.
 
-    $ pip install coreapi
+    $ pip install coreapi pyyaml
 
 We can now include a schema for our API, by including an autogenerated schema
 view in our URL configuration.