docs: change old version hint in docs

Resolves https://github.com/rook/rook.github.io/issues/144

Signed-off-by: Alexander Trost <galexrt@googlemail.com>

.docs/overrides/main.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 
 {% block outdated %}
-  This document is for a development version of Rook.
+  This documentation is not for the latest stable version of Rook.
   <a href="{{ '../' ~ base_url }}/latest-release/">
     <strong>Click here for the latest release documentation.</strong>
   </a>