Properly escape an & sign in epub sources

Issue marijnh#470

Author: marijnh

epub/frontmatter.xhtml

@@ -41,7 +41,7 @@
     and <a href="https://www.ghostery.com/"><img src="img/ghostery_mini.png"
     alt="Ghostery" class="logo"/></a>.</p>
 
-    <p>A <a href="http://a-fwd.com/com=marijhaver-20&asin-com=1593279507">paper
+    <p>A <a href="http://a-fwd.com/com=marijhaver-20&amp;asin-com=1593279507">paper
     version</a> of Eloquent JavaScript, including a bonus chapter, is
     being brought out by <a href="https://nostarch.com/">No Starch
     Press</a>. They also sell a more polished EPUB version that