[release-0.19] Fix typo in pingsource docs

[knative-prow-robot]

This is an automated cherry-pick of #3254

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[abrennan89]

@googlebot I consent.

[mattmoor]

/ok-to-test
/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, mattmoor

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