fix(docs): use 'just' language identifier for syntax highlighting

Change code block language identifier from 'justfile' to 'just' to match
the registered grammar name in just.tmLanguage.json. This resolves the
warning about language not being found during build.

Author: cameronraysmith

packages/docs/src/content/docs/guides/template-usage.md

@@ -232,7 +232,7 @@ matrix:
 
 Update filter patterns:
 
-```justfile
+```just
 # Before
 bun run --filter '@typescript-nix-template/docs' dev