Update typo in comments

flow_metrics database data retention time(unit: hour)
->
flow_log database data retention time(unit: hour)

I think maybe a typo.

Author: Hyzhou

server/server.yaml

@@ -509,7 +509,7 @@ ingester:
   #  vtap-app-1m: 168      # vtap_app[_edge]_port.1m
   #  vtap-app-1s: 24      # vtap_app[_edge]_port.1s
 
-  ## flow_metrics database data retention time(unit: hour)
+  ## flow_log database data retention time(unit: hour)
   ## Note: This configuration is only valid when DeepFlow is run for the first time or the ClickHouse tables have not yet been created
   #flow-log-ttl-hour:
   #  l4-flow-log: 72