Correct eventlistener docs to properly set service account for a trigger

[kscherer]

Changes

When I copied the example from the docs it failed. I followed the other examples and it worked so this updates the docs.

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[tekton-robot]

Hi @kscherer. Thanks for your PR.

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[dibyom]

/ok-to-test

[dibyom]

Thank you!
/approve
/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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