doc: Fix incorrect comment in FlowRuleComparator (#1915)

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/flow/FlowRuleComparator.java

@@ -28,7 +28,7 @@ public class FlowRuleComparator implements Comparator<FlowRule> {
 
     @Override
     public int compare(FlowRule o1, FlowRule o2) {
-        // Clustered mode will be on the top.
+        // the FlowRule in Clustered mode will be put at the end.
         if (o1.isClusterMode() && !o2.isClusterMode()) {
             return 1;
         }