doc: do not use random ip address for dns in default conf

The default configuration has a random ip-address as first DNS server.
This might cause unnedessary trouble for people who try osmo-ggsn the
first time. Lets have some public DNS here, just to be sure.

Change-Id: I5876a806185bb3aea356fb6996d1925b8d0d1758
diff --git a/doc/examples/osmo-ggsn.cfg b/doc/examples/osmo-ggsn.cfg
index 4000636..2bc7668 100644
--- a/doc/examples/osmo-ggsn.cfg
+++ b/doc/examples/osmo-ggsn.cfg
@@ -43,8 +43,8 @@
   tun-device tun4
   type-support v4
   ip prefix dynamic 176.16.222.0/24
-  ip dns 0 192.168.100.1
-  ip dns 1 8.8.8.8
+  ip dns 0 8.8.8.8
+  ip dns 1 8.8.4.4
   ip ifconfig 176.16.222.0/24
   no shutdown
  apn inet6
@@ -61,8 +61,8 @@
   tun-device tun46
   type-support v4v6
   ip prefix dynamic 176.16.46.0/24
-  ip dns 0 192.168.100.1
-  ip dns 1 8.8.8.8
+  ip dns 0 8.8.8.8
+  ip dns 1 8.8.4.4
   ip ifconfig 176.16.46.0/24
   ipv6 prefix dynamic 2001:780:44:2100:0:0:0:0/56
   ipv6 dns 0 2001:4860:4860::8888