Disambiguate "allowed to use" (#707)

There is now another definition in the private aggregation API,
so disambiguate it to the HTML spec.

Fixes a bikeshed warning.

Author: cbiesinger

spec/index.bs

@@ -65,6 +65,7 @@ spec:webdriver2; type:dfn; text:error
 spec:webdriver2; type:dfn; text:remote end steps
 spec:fetch; type:dfn; for:/; text:response
 spec:css-color-5; type:type; text:<color>
+spec:html; type:dfn; text:allowed to use
 </pre>
 
 <style>