COPYING: small spelling fix (bellow -> below)

Author: henningw

COPYING

@@ -12,7 +12,7 @@ IMPORTANT NOTES
    (see http://www.gnu.org/copyleft/gpl-faq.html#GPLAndPlugins
     for a detailed explanation)
 
-3) Note that the GPL bellow is copyrighted by the Free Software Foundation,
+3) Note that the GPL below is copyrighted by the Free Software Foundation,
    but the Kamailio software is copyrighted by multiple individuals and
    companies.