Fix .env examples for PRIVATE_KEY -> GITHUB_PRIVATE_KEY variables

[schlagelk]

Why:

This fixes naming for a couple of variables in the .env examples in this document. Currently there are examples which suggest naming the private key variable PRIVATE_KEY, when really it should be GITHUB_PRIVATE_KEY. When following along in the example and attempting to run the server, using the incorrect name in the .env file leads to an error.

Closes [https://github.com//issues/4304]

What's being changed:

PRIVATE_KEY -> GITHUB_PRIVATE_KEY

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[janiceilene]

Thanks so much for opening an issue and a PR for this change @schlagelk! I'll get this triaged for review ⚡

[hubwriter]

@schlagelk - Thanks so much for correcting this. I hope this error didn't inconvenience you too much when you ran up against it. Very good of you to take time to correct it. Much appreciated. 👍

Thanks again. I'll merge this to get this correction live.

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