fix(standalone-apis): fix broken link to Primer Button CSS

Author: LayZeeDK

docs/standalone-apis/testing-a-standalone-attribute-directive-using-the-angular-testbed.md

@@ -2,7 +2,7 @@
 
 For the purpose of this example, we will create a standalone attribute directive
 that adds the CSS classes provided by [Primer](https://primer.style/) for its
-[Button](https://primer.style/css/components/buttons) component:
+[Button](https://primer.style/components/button/css) component:
 
 ```typescript {12} title="primer-button.directive.ts"
 import { Directive, HostBinding, Input } from "@angular/core";