nat_traversal: switch from deprecated /var/run to /run

  The FHS 3.0 [F] has deprecated /var/run in favor of /run. Current
  distributions have done so for a long time (for example in Debian
  since 6.0 “squeeze”), even though they provide a compatibility
  symlink from /var/run to /run. But software like systemd have
  started emitting warnings when using /var/run, for example for
  its PIDFile directive, which pollutes the logs.

src/modules/nat_traversal/doc/nat_traversal_admin.xml

@@ -492,7 +492,7 @@ modparam("nat_traversal", "keepalive_extra_headers", "User-Agent: &kamailio;\r\n
       <title>Setting the <varname>keepalive_state_file</varname> parameter</title>
         <programlisting format="linespecific">
 ...
-modparam("nat_traversal", "keepalive_state_file", "/var/run/kamailio/keepalive_state")
+modparam("nat_traversal", "keepalive_state_file", "/run/kamailio/keepalive_state")
 ...
         </programlisting>
       </example>