Fix regular time check

We used setTimeout instead of setInterval, so the periodic check was
only performed twice.

Change-Id: I62732483c3d412f1690d01de4a25cdb8249db494
diff --git a/js/script.js b/js/script.js
index 7c97bfd..0a3eef0 100644
--- a/js/script.js
+++ b/js/script.js
@@ -94,7 +94,7 @@
 
           Promise.all(promises).then(booklets => {
             checkBooklets(booklets);
-            window.setTimeout(_ => { checkBooklets(booklets); }, 60 * 1000);
+            window.setInterval(_ => { checkBooklets(booklets); }, 60 * 1000);
           });
         })
         .catch(err => {