docs: update README to reflect CronJobSource renamed to PingSource

[LYSingD]

"Fixes #issue-number" or "Add description of the problem this PR solves"

Proposed Changes

• Update the documentation. Since knative/eventing#2412 updated the resource name from CronJobSource to PingSource, the documentation is misleading;

[linux-foundation-easycla]

• ✅login: LYSingD / (d148363)

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[knative-prow]

Welcome @LYSingD! It looks like this is your first PR to knative/docs 🎉

[knative-prow]

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This pull-request has been approved by: LYSingD
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