Include petsets in kubectl valid commands

Petsets are already implemented in kubectl, but there were no hints
for that subcommand.

Fixes #25615

pkg/kubectl/cmd/cmd.go

@@ -164,6 +164,7 @@ __custom_func() {
    * limitranges (aka 'limits')
    * nodes (aka 'no')
    * namespaces (aka 'ns')
+   * petsets (alpha feature, may be unstable)
    * pods (aka 'po')
    * persistentvolumes (aka 'pv')
    * persistentvolumeclaims (aka 'pvc')