gbproxy: Fixed VTY doc for delete-gbproxy-link

The command definition delete_gb_link_by_id_cmd lacks a description
for the sgsn-nsei token.

This patch adds the missing description.

Addresses:
Verifying src/gprs/osmo-gbproxy -c doc/examples/osmo-gbproxy/osmo-gbproxy.cfg, test verify_doc
Documentation error (missing docs):
<command id='delete-gbproxy-link &lt;0-65534&gt; (tlli|imsi|sgsn-nsei) IDENT'>
        <param name='IDENT' doc='(null)' />

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gb_proxy_vty.c b/openbsc/src/gprs/gb_proxy_vty.c
index 3042cf5..de20e6a 100644
--- a/openbsc/src/gprs/gb_proxy_vty.c
+++ b/openbsc/src/gprs/gb_proxy_vty.c
@@ -626,6 +626,7 @@
 	GBPROXY_DELETE_LINK_STR
 	"Delete entries with a matching TLLI (hex)\n"
 	"Delete entries with a matching IMSI\n"
+	"Delete entries with a matching SGSN NSEI\n"
 	"Identification to match\n")
 {
 	const uint16_t nsei = atoi(argv[0]);