Fix typo in activity query

sample-config/pgmon-metrics.yml

@@ -197,7 +197,7 @@ metrics:
       0: >
         SELECT state,
                count(*) AS backend_count,
-               COALESCE(EXTRACT(EPOCH FROM max(now() - state_change)) 0) AS max_state_time
+               COALESCE(EXTRACT(EPOCH FROM max(now() - state_change)), 0) AS max_state_time
         FROM pg_stat_activity
         WHERE datname = %(dbname)s
         GROUP BY state