chore(backend): typo in max concurrency error text

[Gkrumbach07]

Description of your changes:
Fixes a small typo in error text for max concur out of range error, and related test

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[google-oss-prow]

Hi @Gkrumbach07. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[rimolive]

/lgtm

[HumairAK]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HumairAK

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