.service: remove OpenBSC from service desription

Change-Id: I555939235437a0b3253a1236a918e1f491ed9d3a
diff --git a/contrib/systemd/osmo-sgsn.service b/contrib/systemd/osmo-sgsn.service
index bf6a8e0..0991c70 100644
--- a/contrib/systemd/osmo-sgsn.service
+++ b/contrib/systemd/osmo-sgsn.service
@@ -1,5 +1,5 @@
 [Unit]
-Description=OpenBSC SGSN
+Description=Osmocom SGSN (Serving GPRS Support Node)
 Wants=osmo-hlr.service
 After=osmo-hlr.service
 After=osmo-hnbgw.service