Fix incorrect chapter number in frontmatter

Author: marijnh

epub/frontmatter.xhtml

@@ -23,7 +23,7 @@
     diagrams in Chapter 9 generated
     with <a href="https://regexper.com/">regexper.com</a> by Jeff
     Avallone. Village photograph in Chapter 11 by Fabrice Creuzot.
-    Game concept for Chapter 15
+    Game concept for Chapter 16
     by <a href="http://lessmilk.com">Thomas Palef</a>.</p>
 
     <p style="font-size: 80%">The third edition was made possible

html/index.html

@@ -49,7 +49,7 @@ <h1>Eloquent JavaScript<div style="font-size: 70%">3rd edition</div></h2>
   expression diagrams in Chapter 9 generated
   with <a href="https://regexper.com">regexper.com</a> by Jeff
   Avallone. Village photograph in Chapter 11 by Fabrice Creuzot. Game
-  concept for Chapter 15 by <a href="http://lessmilk.com">Thomas
+  concept for Chapter 16 by <a href="http://lessmilk.com">Thomas
   Palef</a>.</p>
 
   <p>The third edition was made possible

pdf/book.tex

@@ -102,7 +102,7 @@
   7 and 16 by Antonio Perdomo Pastor. Regular expression diagrams in
   Chapter 9 generated with \href{http://regexper.com}{regexper.com} by
   Jeff Avallone. Village photograph in Chapter 11 by Fabrice Creuzot.
-  Game concept for Chapter 15 by \href{http://lessmilk.com}{Thomas
+  Game concept for Chapter 16 by \href{http://lessmilk.com}{Thomas
   Palef}.
 
   The third edition of Eloquent JavaScript was made possible