Add more space around the cover on the front page

Author: marijnh

html/index.html

@@ -15,7 +15,7 @@ <h1>Eloquent JavaScript<div style="font-size: 70%">3rd edition</div></h2>
   copy</a> of the <em>second</em> edition. A paper third edition is
   being worked on, but it isn't clear yet when it'll be available.</p>
 
-  <p style="text-align: center; margin: 1.5em 0">
+  <p style="text-align: center; margin: 3em 0 2.5em">
     <img src="img/cover.jpg" alt="Cover image" style="border: 1px solid black; max-width: 100%; box-shadow: 4px 4px 7px rgba(0, 0, 0, 0.4)">
   </p>