Clarify errata

Author: marijnh

html/errata.html

@@ -96,8 +96,8 @@ <h2>Chapter 17</h2>
 
 <h2>Chapter 18</h2>
 
-<p><strong>Page 316</strong> (Fields): The <code>selected</code>
-attribute in the <code>file</code> at the bottom of the example
+<p><strong>Page 316</strong> (Fields): The <code>checked</code>
+attribute in the <code>file</code> field at the bottom of the example
 doesn't mean anything, and should not be there.</p>
 
 <p><strong>Page 327</strong> (Summary¹): “When the user has selected