MINOR Fix counter in mock SR client

[rayokota]

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[cla-assistant]

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1 out of 2 committers have signed the CLA.

✅ rayokota
❌ ConfluentJenkins
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ rayokota
❌ ConfluentJenkins
_{You have signed the CLA already but the status is still pending? Let us recheck it.}