docs: add missing signal function (#63974)

The `buttonClasses` variable should be a signal according to the example, so we added the call.
PR Close #63974

adev/src/content/guide/templates/binding.md

@@ -151,7 +151,7 @@ You can also bind directly to the `class` property. Angular accepts three types
 export class UserProfile {
   listClasses = 'full-width outlined';
   sectionClasses = signal(['expandable', 'elevated']);
-  buttonClasses = ({
+  buttonClasses = signal({
     highlighted: true,
     embiggened: false,
   });