Simplify CLA signing instructions to only mention comment mechanism

[nhorton]

No description provided.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Developer Certificate of Origin before we can accept your contribution. You can sign the DCO by just posting a Pull Request Comment same as the below format.

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I have read the CLA Document and I hereby sign the CLA

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_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the DCO Assistant Lite bot.}