Update testing instructions to use gateway run

Change testing instructions from "configure and test your remote server
in the MCP Toolkit on Docker Desktop" to "start the gateway with
docker mcp gateway run and test tool calls to the remote server"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: kgprs

CONTRIBUTING.md

@@ -320,7 +320,7 @@ For OAuth-enabled servers, authorize the server:
 docker mcp oauth authorize my-remote-server
 ```
 
-Now you can configure and test your remote server in the MCP Toolkit on Docker Desktop.
+Now you can start the gateway with `docker mcp gateway run` and test tool calls to the remote server.
 
 When done testing, reset the catalog: