debian: fix name of config file

Change-Id: I9e563d41b74c334fd98bb71fad946e93f214fa34
diff --git a/debian/osmo-ggsn.examples b/debian/osmo-ggsn.examples
index 775b9cd..fe0136c 100644
--- a/debian/osmo-ggsn.examples
+++ b/debian/osmo-ggsn.examples
@@ -1,2 +1,2 @@
-doc/examples/osmo-ggsn.conf
+doc/examples/osmo-ggsn.cfg
 doc/examples/sgsnemu.conf