.github/workflows: let prettier format yml files

I don't care. But now when I hit save it won't change any more.
diff --git a/.github/workflows/formatting.yml b/.github/workflows/formatting.yml
index 333365a..b4dc41e 100644
--- a/.github/workflows/formatting.yml
+++ b/.github/workflows/formatting.yml
@@ -29,8 +29,8 @@
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '20'
-          cache: 'npm'
+          node-version: "20"
+          cache: "npm"
           cache-dependency-path: webui/package-lock.json
 
       - name: Install dependencies