Add tabindex attribute to disabled buttons sample. Closes #23361 (#25680)

As discussed in issue's comment:
https://git.io/vAuPW

Thanks!

docs/4.0/components/buttons.md

@@ -90,8 +90,8 @@ Disabled buttons using the `<a>` element behave a bit different:
 - Disabled buttons should include the `aria-disabled="true"` attribute to indicate the state of the element to assistive technologies.
 
 {% example html %}
-<a href="#" class="btn btn-primary btn-lg disabled" role="button" aria-disabled="true">Primary link</a>
-<a href="#" class="btn btn-secondary btn-lg disabled" role="button" aria-disabled="true">Link</a>
+<a href="#" class="btn btn-primary btn-lg disabled" tabindex="-1" role="button" aria-disabled="true">Primary link</a>
+<a href="#" class="btn btn-secondary btn-lg disabled" tabindex="-1" role="button" aria-disabled="true">Link</a>
 {% endexample %}
 
 {% callout warning %}