Fix extra closing brace at end of JSON example in Chapter 21

Closes marijnh#491

Author: marijnh

21_skillsharing.md

@@ -160,7 +160,7 @@ A `GET` request to `/talks` returns a JSON document like this:
 [{"title": "Unituning",
   "presenter": "Jamal",
   "summary": "Modifying your cycle for extra style",
-  "comments": []}]}
+  "comments": []}]
 ```
 
 {{index "PUT method", URL}}

html/errata.html

@@ -84,6 +84,11 @@ <h2>Chapter 20</h2>
 Creation</em>: <code>MKCOL</code> stands for “make collection”, not “make
 column” as the book claims.</p>
 
+<h2>Chapter 21</h2>
+
+<p><strong>Page 373</strong> <em>HTTP Interface</em>: There is a
+superfluous closing brace at the end of the example JSON snippet.</p>
+
 <h2>Exercise Hints</h2>
 
 <p><strong>Page 414</strong> <em>A Modular Robot</em>: