Updates to GKE Terraform docs

[joeholley]

What type of PR is this?

/kind documentation

What this PR does / Why we need it:
Simple typo/grammar mistakes

Which issue(s) this PR fixes:

N/A

[agones-bot]

Build Failed 😱

Build Id: 8ecd6599-c7be-474d-a4e1-b898c4928f37

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[markmandel]

Thanks!

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: joeholley, markmandel

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• OWNERS [markmandel]

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[google-oss-prow]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Succeeded 👏

Build Id: a3321f63-8e58-48f8-af9d-150313e7bdc0

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.35.0-dev-d8c5e80-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.35.0-dev-d8c5e80-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.35.0-dev-d8c5e80-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.35.0-dev-d8c5e80-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.35.0-dev-d8c5e80-amd64
• Linux C++ SDK (build): agonessdk-1.35.0-dev-d8c5e80-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.35.0-dev-d8c5e80-amd64.zip

A preview of the website (the last 30 builds are retained):

• https://d8c5e80-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3360/head:pr_3360 && git checkout pr_3360
• helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.35.0-dev-d8c5e80-amd64