errata: format string on p.200

Change-Id: Iba865c44a2d86a01dfd3e75a629e2abfa54d53a9

errata.html

@@ -96,6 +96,13 @@ pre {
     <b>p.166, ¶2</b>: for "way", read "a way".
   </p>
 
+  <p>
+    <b>p.200, <code>TestEval</code> function</b>: the format string in the final call to
+    <code>t.Errorf</code> should format <code>test.env</code> with <code>%v</code>,
+    not <code>%s</code>.
+    (Thanks to Mitsuteru Sawa, 2015-12-07.)
+  </p>
+
   <p>
     <b>p.362</b>: the <code>gopl.io/ch13/bzip</code> program
     does not comply with the