Update role-supports-aria-props.md: fix code sample

fixes jsx-eslint#938

Author: n-chardon

docs/rules/role-supports-aria-props.md

@@ -13,7 +13,7 @@ This rule takes no arguments.
 ### Succeed
 ```jsx
 <!-- Good: the radiogroup role does support the aria-required property -->
-<ul role="radiogroup" aria-required aria-labelledby="foo">
+<ul role="radiogroup" aria-required="true" aria-labelledby="foo">
     <li tabIndex="-1" role="radio" aria-checked="false">Rainbow Trout</li>
     <li tabIndex="-1" role="radio" aria-checked="false">Brook Trout</li>
     <li tabIndex="0" role="radio" aria-checked="true">Lake Trout</li>