Add dijkstrajs typo to errata

Issue marijnh#488

Author: marijnh

html/errata.html

@@ -75,4 +75,10 @@ <h2>Chapter 20</h2>
 Creation</em>: <code>MKCOL</code> stands for “make collection”, not “make
 column” as the book claims.</p>
 
+<h2>Exercise Hints</h2>
+
+<p><strong>Page 414</strong> <em>A Modular Robot</em>:
+The <code>dijkstrajs</code> package name is misspelled
+as <code>dijkstajs</code>.
+
 </article>