Zookeeper: fix misspelled purgeInterval variable

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[realdimas]

/assign @janetkuo

[realdimas]

/unassign /assign kow3ns

[kow3ns]

/approve

[kow3ns]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: forkbomber, kow3ns

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