Corrected gke docs 'release-release' typo

[eddie-knight]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug
/kind cleanup

/kind documentation

/kind feature
/kind hotfix

What this PR does / Why we need it:

A small typo existed in an installation step, so this PR corrects for that.

The current text is as follows:

For example, if you are targeting release release-1.8.0, then you will want to have source = "git::https://github.com/googleforgames/agones.git//install/terraform/modules/gke/?ref=release-release-1.8.0" as your source.

This should instead say ?ref=release-1.8.0.

Which issue(s) this PR fixes:

Special notes for your reviewer:

[agones-bot]

Build Failed 😱

Build Id: 0d7717e1-085e-4b0e-b4d9-e048b1888441

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[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: eddie-knight, roberthbailey

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[agones-bot]

Build Failed 😱

Build Id: c38eb230-0686-4fe8-acf7-8245fd8d9511

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[roberthbailey]

Flake in unrelated tests:

--- FAIL: TestFleetScaleUpEditAndScaleDown (0.03s)
    --- FAIL: TestFleetScaleUpEditAndScaleDown/Use_fleet_Patch_false (362.96s)
        fleet_test.go:192: 
            	Error Trace:	fleet_test.go:192
            	Error:      	Expected nil, but got: &errors.errorString{s:"timed out waiting for the condition"}
            	Test:       	TestFleetScaleUpEditAndScaleDown/Use_fleet_Patch_false

[agones-bot]

Build Succeeded 👏

Build Id: 5667d0c0-1eec-4853-9efb-17f206feaede

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

• image: gcr.io/agones-images/agones-controller:1.9.0-94e3bf1
• image: gcr.io/agones-images/agones-sdk:1.9.0-94e3bf1
• image: gcr.io/agones-images/agones-ping:1.9.0-94e3bf1
• Linux C++ SDK (build): agonessdk-1.9.0-94e3bf1-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.9.0-94e3bf1.zip

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

• https://94e3bf1-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1826/head:pr_1826 && git checkout pr_1826
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.9.0-94e3bf1