Proposed edits to oidc-github guide

[EronWright]

Description

Minor corrections and a suggestion that the authz rule be based on repository or sub as outlined here.

TODO: update the picture depicting the addition of a rule.

Checklist:

• [ ] I have reviewed the style guide.
• [ ] If blogging, I have reviewed the blogging guide.
• [ ] I have manually confirmed that all new links work.
• [ ] I added aliases (i.e., redirects) for all filename changes.
• [ ] If making css changes, I rebuilt the bundle.

[pulumi-bot]

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