Check for operating repos doesn't normalize to lowercase

I just installed this for a repo with an organization whose name contains mixed case. I did not think to mix the case when I provided the `GITHUB_REPO_NAME` environment variable. This cased a 403 error with an error of `{"message":"Not configured for this repository"}`. I updated the organization's name in the environment variable to match the case as found in the payload and then the webhook succeeded.

Recommend normalizing both `repoSplit[0]` and `checkSplit[0]` to lowercase for comparison.

https://github.com/helm/pull-sizer/blob/bb557b11dfb698c211d6a9dd70935c93a97642ee/main.go#L176