reword schedule to scheduling in KEP 3521

Author: Huang-Wei

keps/sig-scheduling/3521-pod-schedule-readiness/README.md renamed to keps/sig-scheduling/3521-pod-scheduling-readiness/README.md

@@ -58,7 +58,7 @@ If none of those approvers are still appropriate, then changes to that list
 should be approved by the remaining approvers and/or the owning SIG (or
 SIG Architecture for cross-cutting KEPs).
 -->
-# KEP-3521: Pod Schedule Readiness
+# KEP-3521: Pod Scheduling Readiness
 
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