Fix typo in pingsource docs

[abrennan89]

Fixes #3154

[mattmoor]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• docs/eventing/sources/OWNERS [abrennan89,mattmoor]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[abrennan89]

/cherrypick release-0.18

[knative-prow-robot]

@abrennan89: new pull request created: #3255

In response to this:

/cherrypick release-0.18

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.

[abrennan89]

/cherrypick release-0.19

[knative-prow-robot]

@abrennan89: new pull request created: #3256

In response to this:

/cherrypick release-0.19

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.

[abrennan89]

/cherrypick release-0.20

[knative-prow-robot]

@abrennan89: new pull request created: #3257

In response to this:

/cherrypick release-0.20

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.