fix bug in heading id (#210)

Author: wip-abramson

index.html

@@ -192,7 +192,7 @@ <h1>Introduction</h1>
 	is being thoroughly discussed by the community. For example, see
 	<a href="https://github.com/w3c/did-spec/issues/166#issuecomment-464502719">this comment</a>.</p>
 
-	<section id="design-goals" class="informative">
+	<section id="implementer-overview" class="informative">
 		<h2>Implementer Overview</h2>
 		<p>
 			By invoking a <a>DID resolver</a> using the standard <code>resolve(did, resolutionOptions)</code> interface (as defined in