fix(backend): Fix typo in jwk-remote-missing error message

[dimkl]

Description

Fix typo

Checklist

• npm test runs as expected.
• npm run build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

[changeset-bot]

🦋 Changeset detected

Latest commit: 797a168

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@clerk/backend	Patch
@clerk/fastify	Patch
gatsby-plugin-clerk	Patch
@clerk/nextjs	Patch
@clerk/remix	Patch
@clerk/clerk-sdk-node	Patch

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[LekoArts]

It can be a single kid or multiple kid since it's an array that gets concenated to a comma-delimited list. That's why I chose are - if we want to have the pluralization correct all the time we should use is / are depending on how many items the cacheValues array has.