Fix extension of template file (sysadmin#17839)

Bug introduced in: 201e92a3

manifests/vpn/instance.pp

@@ -90,7 +90,7 @@ define tails::vpn::instance (
       owner   => 'root',
       group   => 'root',
       mode    => '0600',
-      content => epp('tails/vpn/tinc.conf.erb', {
+      content => epp('tails/vpn/tinc.conf.epp', {
         hostname   => $hostname,
         connect_to => $connect_to,
         proxy      => $proxy,