chore: update mock server docs

Author: stainless-app[bot]

CONTRIBUTING.md

@@ -85,11 +85,10 @@ $ pip install ./path-to-wheel-file.whl
 
 ## Running tests
 
-Most tests require you to [set up a mock server](https://github.com/stoplightio/prism) against the OpenAPI spec to run the tests.
+Most tests require you to [set up a mock server](https://github.com/dgellow/steady) against the OpenAPI spec to run the tests.
 
 ```sh
-# you will need npm installed
-$ npx prism mock path/to/your/openapi.yml
+$ ./scripts/mock
 ```
 
 ```sh