Fix the documentation on github-runner.md to use correct variable name for githubApiURL

[zerola]

The variable name should be githubApiURL, not the githubAPIURL as per the line below:

https://github.com/kedacore/keda/blob/main/pkg/scalers/github_runner_scaler.go#L412

This PR extends the PR #1261 for other versions of above mentioned issue as was requested, but activity on the PR got stalled.
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Fixes #

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[zroubalik]

LGTM, thanks!