Fixed docs for compute instance templates

[kvanzuijlen]

The current docs give incorrect instructions for the scopes field in the compute instance template resources. This PR aims to fix those incorrect instructions.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

computeengine: improved documentation for compute engine instance templates oauth scopes 

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[modular-magician]

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[bschaatsbergen]

Happy to see you contributing to magic modules @kvanzuijlen - looking forward to more PRs 👍🏼🎉

[rileykarson]

LGTM- this is consistent with the guidance in google_compute_instance: https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_instance#scope. They used to say different things!

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 8 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 2 files changed, 8 insertions(+), 4 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[bschaatsbergen]

Thanks for picking this up so fast, much appreciated. Happy to see that several of my colleagues started actively contributing 😎