fix(button): update demo xl variant class to xlarge

src/plugins/components/button/button.variants.ts

@@ -4,7 +4,7 @@ export const size = {
   sm: 'nui-button-small',
   md: 'nui-button-medium',
   lg: 'nui-button-large',
-  xl: 'nui-button-big',
+  xl: 'nui-button-xlarge',
 } as const satisfies ButtonVariant<'size'>
 
 export const rounded = {