[skip ci] Refine Describe Index Rate panel description (#10762)

Signed-off-by: NEdward Zeng <jie.zeng@zilliz.com>

deployments/monitor/grafana/milvus-dashboard.json

@@ -2557,7 +2557,7 @@
       "dashLength": 10,
       "dashes": false,
       "datasource": "$datasource",
-      "description": "per-second rate of describe_index as measured over the last 5 minutes",
+      "description": "per-minute rate of describe index",
       "fieldConfig": {
         "defaults": {},
         "overrides": []