stats: Document the level in show stats level options

Potentially Fixes:

<command id='show stats level (global|peer|subscriber)'>
        <param name='subscriber' doc='(null)' />
diff --git a/src/vty/stats_vty.c b/src/vty/stats_vty.c
index feda2aa..e0239bf 100644
--- a/src/vty/stats_vty.c
+++ b/src/vty/stats_vty.c
@@ -344,6 +344,7 @@
       show_stats_level_cmd,
       "show stats level (global|peer|subscriber)",
       SHOW_STR SHOW_STATS_STR
+      "Set the maximum group level\n"
       "Show global groups only\n"
       "Show global and network peer related groups\n"
       "Show global, peer, and subscriber groups\n")