Update helm installation to include a step to update helm repo

[pooneh-m]

Add helm repo update to the instruction to update the helm

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 documentation

What this PR does / Why we need it:
When a new version of Agones is released, in order to use the new helm configurations, helm repo needs to get updated. Adding Agones chart to helm, if already available, does not update the chart.

[agones-bot]

Build Succeeded 👏

Build Id: 11ce83a9-8fb2-4d4d-94b3-ae634ab7278f

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

• image: gcr.io/agones-images/agones-controller:1.10.0-46af693
• image: gcr.io/agones-images/agones-sdk:1.10.0-46af693
• image: gcr.io/agones-images/agones-ping:1.10.0-46af693
• Linux C++ SDK (build): agonessdk-1.10.0-46af693-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.10.0-46af693.zip

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

• https://46af693-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1881/head:pr_1881 && git checkout pr_1881
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.10.0-46af693

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pooneh-m, roberthbailey

The full list of commands accepted by this bot can be found here.

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• OWNERS [pooneh-m,roberthbailey]

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[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Failed 😱

Build Id: 807e6532-7c76-4d3a-afdb-f67d268b7814

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• Cloud Build log download

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[agones-bot]

Build Succeeded 👏

Build Id: d7681194-425b-4f42-a120-b853d928a35f

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

• image: gcr.io/agones-images/agones-controller:1.10.0-7855b0c
• image: gcr.io/agones-images/agones-sdk:1.10.0-7855b0c
• image: gcr.io/agones-images/agones-ping:1.10.0-7855b0c
• Linux C++ SDK (build): agonessdk-1.10.0-7855b0c-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.10.0-7855b0c.zip

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

• https://7855b0c-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1881/head:pr_1881 && git checkout pr_1881
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.10.0-7855b0c