Update DestinationRule docs with correct env variable name

[brobb7]

The Destination Rule documentation show that the verify cert at client environment variable name is VERIFY_CERT_AT_CLIENT in ClientTLSSettings but according to the pilot-discovery docs the environment variable is VERIFY_CERTIFICATE_AT_CLIENT. Updating the Destination Rule documentation to use the correct environment variable name.

[istio-policy-bot]

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[ericvn]

Nit:

Suggested change

	**Fixed** Incorrect pilot-discovery environment variable name.
	**Fixed** incorrect pilot-discovery environment variable name.

[brobb7]

Fixed this.

[jacob-delgado]

/cherry-pick release-1.16

[istio-testing]

@jacob-delgado: new pull request created: #2597

In response to this:

/cherry-pick release-1.16

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[istio-testing]

@jacob-delgado: new pull request could not be created: failed to create pull request against istio/api#release-1.16 from head istio-testing:cherry-pick-2596-to-release-1.16: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for istio-testing:cherry-pick-2596-to-release-1.16."}],"documentation_url":"https://docs.github.com/rest/reference/pulls#create-a-pull-request"}

In response to this:

/cherry-pick release-1.16

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.