Fix a typo in Chapter 18

Author: marijnh

18_forms.txt

@@ -765,7 +765,7 @@ handler can call `preventDefault` on that event to prevent the submission from
 happening. Form field elements do not have to be wrapped in `<form>`
 tags.
 
-When the user has selected a field from their local file system in a
+When the user has selected a file from their local file system in a
 file picker field, the `FileReader` interface can be used to access
 the content of this file from a JavaScript program.
 

html/errata.html

@@ -74,4 +74,10 @@ <h2>Chapter 17</h2>
 the <code>statusCode</code> property. This does not exist, since the
 property is actually called <code>status</code>.</p>
 
+<h2>Chapter 18</h2>
+
+<p><strong>Page 327</strong> (Summary): “When the user has selected
+a <em>field</em>” should be “When the user has selected
+a <em>file</em>”.
+
 </article>