docs: fix reading time tracking :(

docs/_templates/layout.html

@@ -10,6 +10,7 @@
       _paq.push(['enableLinkTracking']);
       (function() {
         var u="https://k1.botanicus.net/tr/";
+        _paq.push(['enableHeartBeatTimer', 30]);
         _paq.push(['setTrackerUrl', u+'ep']);
         _paq.push(['setSiteId', 6]);
         var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0]; g.type='text/javascript';