Change allocator's preferredGameServerSelector field name to plural

[pooneh-m]

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This pull-request has been approved by: markmandel, pooneh-m
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[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Succeeded 👏

Build Id: f7d7c1b3-c3cc-4fac-9cc9-51457c369812

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

• image: gcr.io/agones-images/agones-controller:1.0.0-1a64b68
• image: gcr.io/agones-images/agones-sdk:1.0.0-1a64b68
• image: gcr.io/agones-images/agones-ping:1.0.0-1a64b68
• Linux C++ SDK (build): agonessdk-1.0.0-1a64b68-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.0.0-1a64b68.zip

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

• https://1a64b68-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1047/head:pr_1047 && git checkout pr_1047
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.0.0-1a64b68