doc: fix homepage icon

The icon did not display properly on https://shaarli.readthedocs.io/en/master/

doc/md/index.md

@@ -1,4 +1,4 @@
-# <img src="../../assets/default/img/favicon.png" width="20px" height="20px"> Shaarli
+# <img src="images/icon.png" width="20px" height="20px"> Shaarli
 
 The personal, minimalist, super-fast, database free, bookmarking service.