Fix typo in parameter name in chapter 21

Author: marijnh

21_skillsharing.txt

@@ -593,7 +593,7 @@ router.add("GET", /^\/talks$/, function(request, response) {
 When the `changesSince` parameter is missing, the handler simply
 builds up a list of all talks and returns that.
 
-(((long polling)))(((validation)))Otherwise, the `changeSince`
+(((long polling)))(((validation)))Otherwise, the `changesSince`
 parameter first has to be checked to make sure that it is a valid
 number. The `getChangedTalks` function, to be defined shortly, returns
 an array of changed talks since a given point in time. If it returns an

html/errata.html

@@ -109,4 +109,10 @@ <h2>Chapter 18</h2>
 a <em>field</em>” should be “When the user has selected
 a <em>file</em>”.
 
+<h2>Chapter 21</h2>
+
+<p><strong>Page 377</strong> (Long-Polling Support): In the second
+paragraph the <code>changesSince</code> parameter is incorrectly
+referred to as <code>changeSince</code> (singular).</p>
+
 </article>