Fix typeface sample for Courier in index

The example for Courier in Typefaces was using `code` instead of
`courier` for its class.

Author: pedrofialho

index.html

@@ -472,8 +472,8 @@ <h4 class="f6 mb0 mt3">Code</h4>
       <p class="code"> a b c d e f g h i j k l m n o p q r s t u v w x y z</p>
       <p class="code ttu"> a b c d e f g h i j k l m n o p q r s t u v w x y z</p>
 <h4 class="f6 mb0 mt3">Courier</h4>
-      <p class="code"> a b c d e f g h i j k l m n o p q r s t u v w x y z</p>
-      <p class="code ttu"> a b c d e f g h i j k l m n o p q r s t u v w x y z</p>
+      <p class="courier"> a b c d e f g h i j k l m n o p q r s t u v w x y z</p>
+      <p class="courier ttu"> a b c d e f g h i j k l m n o p q r s t u v w x y z</p>
     </div>
     <div class="dtc-ns v-mid">
 <pre class="ba b--black-05  pa2 overflow-auto">