updated error page

A remark about the search function was added to the "page not found" page.

Author: JoramSoch

404.html

@@ -26,5 +26,6 @@ <h1>This page doesn't exist!</h1>
   <ul>
 	<li>If the URL reads <code class="language-plaintext highlighter-rouge">.../P/[shortcut]</code>, create a <a href="/contribute/">new page</a> using the <a href="https://raw.githubusercontent.com/StatProofBook/StatProofBook.github.io/master/P/-temp-.md">proof template</a>.</li>
     <li>If the URL reads <code class="language-plaintext highlighter-rouge">.../D/[shortcut]</code>, create a <a href="/contribute/">new page</a> using the <a href="https://raw.githubusercontent.com/StatProofBook/StatProofBook.github.io/master/D/-temp-.md">definition template</a>.</li>
+	<li>Alternatively, you may also use the <a href="/search">search</a> function of the website.</li>
   </ul>
 </div>