doc: fix form background value (#50)

Author: ouvreboite

styles/forms/form.html

@@ -6,7 +6,7 @@ <h2 id="form">
     Has rounded corners, a background, and an automatic overflow.
   </p>
   <ul>
-    <li><code data-hl="css">code</code> has a <code>◉ --bg-subtle</code> background.</li>
+    <li><code data-hl="css">code</code> has a <code>◉ --bg-muted</code> background.</li>
   </ul>
   <div class="example">
     <form method="get" onsubmit="event.preventDefault()">