chore: update mock server docs

Author: stainless-app[bot]

CONTRIBUTING.md

@@ -68,7 +68,7 @@ $ pnpm link -—global orb-billing
 Most tests require you to [set up a mock server](https://github.com/stoplightio/prism) against the OpenAPI spec to run the tests.
 
 ```sh
-$ npx prism mock path/to/your/openapi.yml
+$ ./scripts/mock
 ```
 
 ```sh