Update link in guidelines.html

The most recent guidelines are not from 2013 but 2020. Whether they're
current (as the text in the file says) is another matter entirely.

guidelines.html

@@ -6,6 +6,6 @@
    <TITLE>Current IOCCC Guidelines</TITLE>
 </HEAD>
 <BODY TEXT="#000000">
-<P>The <A HREF="/2013/guidelines.txt">current guidelines</A></P>
+<P>The <A HREF="/2020/guidelines.txt">current guidelines</A></P>
 </BODY>
 </HTML>