docs: fix incorrect filename in nextjs auth example README

[oscr]

I have read the CONTRIBUTING.md file.

YES

What kind of change does this PR introduce?

The auth nextjs README refers to .env.local.example instead .env.example. This commit updates the README with the correct filename.

What is the current behavior?

N/A

What is the new behavior?

N/A

Additional context

N/A

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