Correct the message for "step" failed in "taskrun"

[vincent-pli]

Fix issue: #2518

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[dibyom]

/approve

[afrittoli]

Nice catch, thank you
/approve

[vdemeester]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: afrittoli, dibyom, vdemeester

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[vincent-pli]

/test pull-tekton-pipeline-integration-tests

[vincent-pli]

/test pull-tekton-pipeline-integration-tests

[vdemeester]

/lgtm

[vincent-pli]

/retest