Fix #357 link to index.html (#375)

Author: JackDunnCodes

navigation-api/subpage.html

@@ -20,7 +20,7 @@ <h1>
         navigation, in browsers that support the navigation API!
       </p>
 
-      <p><a href="/navigation-api/">Back to index.html</a>.</p>
+      <p><a href="index.html">Back to index.html</a>.</p>
 
       <p><button onclick="history.back()">history.back()</button></p>
     </main>