Update pd-scheduling.md

Signed-off-by: anotherrachel <[email protected]>

content/docs/3.0/tasks/best-practices/pd-scheduling.md

@@ -10,6 +10,7 @@ menu:
 This document details the principles and strategies of PD scheduling through common scenarios to facilitate your application. This document assumes that you have a basic understanding of TiDB, TiKV and PD with the following core concepts:
 
 - [leader/follower/learner](../../concepts/glossary.md#leaderfollowerlearner)
+- [operator](../../concepts/glossary.md#operator)
 - [operator step](../../concepts/glossary.md#operator-step)
 - [pending/down](../../concepts/glossary.md#pendingdown)
 - [region/peer/Raft group](../../concepts/glossary.md#regionpeerraft-group)