Fix tomli-w package name in flake.nix

Changed tomli-w to tomli_w to match nixpkgs Python package naming
convention. Nix Python packages use underscores instead of hyphens
in their attribute names.

Author: claude

flake.nix

@@ -42,7 +42,7 @@
             jinja2
             pydantic
             pyyaml
-            tomli-w
+            tomli_w
           ];
 
           doCheck = false;