Fix missing attribute in role-required-states example (act-rules#2139)

Co-authored-by: Carlos Duarte <caduarte@campus.ul.pt>

Author: WilcoFiers

_rules/role-required-states-and-properties-4e8ab6.md

@@ -78,7 +78,7 @@ This `heading` has the required `aria-level` property.
 This `checkbox` has the required `aria-checked` property.
 
 ```html
-<div role="checkbox" aria-checked="false"></div>
+<div role="checkbox" aria-checked="false" aria-labelledby="label"></div>
 <div id="label">Check me</div>
 ```