Add errata entry for promtDirection typo

See marijnh#567

Author: marijnh

html/errata.html

@@ -52,6 +52,10 @@ <h2>Chapter 8</h2>
 called <code>promptNumber</code>, and the word “whole” should be
 dropped from the sentence (it accepts non-whole numbers, too).</p>
 
+<p><strong>Page 139</strong> <em>Selective Catching</em>: The first
+code snippet misspells <code>promptDirection</code>
+as <code>promtDirection</code>.</p>
+
 <h2>Chapter 10</h2>
 
 <p><strong>Page 168</strong> (1st) <em>Modules as Building Blocks</em>: In “each