Update Http Passthrough Auth Doc

[lamadome]

Description

The doc for Http Passthrough Auth is creating a service with the wrong name for the example auth service.
It uses example-grpc-auth-service instead of example-http-auth-service

Context

Users ran into this bug doing ... \ Users needed this feature to ...

Checklist:

• I included a concise, user-facing changelog (for details, see https://github.com/solo-io/go-utils/tree/main/changelogutils) which references the issue that is resolved.
• If I updated APIs (our protos) or helm values, I ran make -B install-go-tools generated-code to ensure there will be no code diff
• I followed guidelines laid out in the Gloo Edge contribution guide
• I opened a draft PR or added the work in progress label if my PR is not ready for review
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[github-actions]

Visit the preview URL for this PR (updated for commit d859c9f):

https://gloo-edge--pr8483-fix-extauthdoc-jona-9ibyc596.web.app

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[jenshu]

/skip-changelog