Add the first errata for the 3rd edition

Author: marijnh

html/errata.html

@@ -21,6 +21,10 @@ <h1>Errata<div style="font-size: 70%"><a class="subtlelink" href="index.html">El
 page</a>. To report a problem that is not listed
 here, <a href="mailto:marijnh@gmail.com">send me an email</a>.</p>
 
-<p><em>No errata yet, since the book isn't in print yet.</em></p>
+
+<h2>Chapter 2</h2>
+
+<p><strong>Page 34</strong> (Updating Bindings Succintly): Where it
+says <code>counter-</code> it should be <code>counter--</code>.</p>
 
 </article>