Adjust the cover image size on mobile

Limit the image maximum width to avoid horizontal scrolling.

Author: atw

html/index.html

@@ -14,7 +14,7 @@ <h1>Eloquent JavaScript<div style="font-size: 70%">second edition</div></h2>
   <a href="http://www.amazon.com/gp/product/1593275846/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1593275846&linkCode=as2&tag=marijhaver-20&linkId=VPXXXSRYC5COG5R5">paperback copy</a> of the book.</p>
 
   <p style="text-align: center; margin: 1.5em 0">
-    <a href="http://www.amazon.com/gp/product/1593275846/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1593275846&linkCode=as2&tag=marijhaver-20&linkId=VPXXXSRYC5COG5R5"><img src="img/cover.png" alt="Cover image" style="border: 1px solid black"></a>
+    <a href="http://www.amazon.com/gp/product/1593275846/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1593275846&linkCode=as2&tag=marijhaver-20&linkId=VPXXXSRYC5COG5R5"><img src="img/cover.png" alt="Cover image" style="border: 1px solid black; max-width: 100%;"></a>
   </p>
 
   <p>Written by Marijn Haverbeke.</p>