docs: Update error display for password field in signal forms playground

Change error message display from paragraph to list format in signal forms playground.

adev/src/content/tutorials/playground/4-signal-forms/src/main.ts

@@ -33,11 +33,11 @@ interface LoginData {
         </label>
 
         @if (loginForm.password().invalid()) {
-          <div class="error">
+          <ul class="error-list">
             @for (error of loginForm.password().errors(); track error) {
-              <p>{{ error.message }}</p>
+              <li>{{ error.message }}</li>
             }
-          </div>
+          </ul>
         }
       </div>