contexts: refer to HTML non-normatively in note

We cannot use a normative reference to HTML in a non-normative
section, such as a note.  This fixes a ReSpec error caused by moving
the index to the end of the document.

index.html

@@ -3645,11 +3645,11 @@ <h2>Contexts</h2>
   </dl>
 </ol>
 
-<p class=note>In accordance with
- the <a href=https://html.spec.whatwg.org/multipage/interaction.html#focus>focus</a>
- section of the [[!HTML]] specification,
- commands are unaffected by whether
- the operating system window has focus or not.
+<p class=note>
+In accordance with
+the <a href=https://html.spec.whatwg.org/multipage/interaction.html#focus>focus</a>
+section of the [[HTML]] specification,
+commands are unaffected by whether the operating system window has focus or not.
 
 <section>
 <h3><dfn>Get Window Handle</dfn></h3>