config-tracing configmap fix

[Shashankft9]

"Fixes #issue-number" or "Add description of the problem this PR solves"
Noticed that the key enable is not even in the structure of tracing config:

type Config struct {
    Backend              BackendType
    ZipkinEndpoint       string
    StackdriverProjectID string
    Debug                bool
    SampleRate           float64
}

also noticed here: knative/eventing#1889 (comment)
also its not mentioned in the configuration options in the doc, so I figured it might be something left over from previous versions. Let me know if it really is required?

Proposed Changes

• just replaced enable key with backend

[knative-prow-robot]

Hi @Shashankft9. Thanks for your PR.

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[abrennan89]

/ok-to-test
/hold

[matzew]

/lgtm
/approve

[abrennan89]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abrennan89, matzew

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[abrennan89]

/unhold