Update README example

Author: edmundmiller

README.md

@@ -45,7 +45,9 @@ jobs:
       - uses: actions/checkout@v3
       - uses: DeterminateSystems/nix-installer-action@main
       - uses: DeterminateSystems/magic-nix-cache-action@main
-      - run: nix flake check
+      - uses: DeterminateSystems/flake-checker-action@main
+      - name: Run `nix build`
+        run: nix build .
 ```
 
 That's it.