gbproxy: The "[stats]" option was not documented, document it

Documentation error (missing docs):
<command id='show gbproxy [stats]'>
        <param name='[stats]' doc='(null)' />
diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index f900158..3c41529 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -931,7 +931,7 @@
 }
 
 gDEFUN(show_gbproxy, show_gbproxy_cmd, "show gbproxy [stats]",
-       SHOW_STR "Display information about the Gb proxy")
+       SHOW_STR "Display information about the Gb proxy\n" "Show statistics\n")
 {
 	struct gbprox_peer *peer;
 	int show_stats = argc >= 1;