Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -48,7 +48,8 @@ target.
 * (Only the first time) `./build/install-build-deps.sh`
 * `./sky/tools/gn`
 * `ninja -C out/Debug`
-* `./mojo/tools/mojodb start out/Debug sky/sdk/example/hello_world/lib/main.dart`
+* `./out/Debug/sky_shell --package-root=out/Debug/gen/dart-pkg/packages sky/sdk/example/hello_world/lib/main.dart`
+  Note: Because Linux is headless, you won't see any UI. You can use `print` to generate console output or you can interact with the DartVM via observatory at [http://localhost:8181/](http://localhost:8181/).
 
 Contributing code
 -----------------