add erratum

errata.html

@@ -23,5 +23,10 @@ pre {
     each time it is run. (Thanks to Randall McPherson.)
   </p>
   <pre>rand.Seed(time.Now().UTC().UnixNano())</pre>
+
+  <p>
+    <b>p.9, paragraph 2</b>: for "can compared", read "can be compared".
+    (Thanks to Antonio Macías Ojeda.)
+  </p>
 </body>
 </html>