Fix typo on metrics documentation

[snowfenix]

No description provided.

[googlebot]

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[snowfenix]

I signed it!

[googlebot]

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[agones-bot]

Build Succeeded 👏

Build Id: b7342e1e-211d-457b-b061-82b5a65e3b26

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

• image: gcr.io/agones-images/agones-controller:0.9.0-7d53bcc
• image: gcr.io/agones-images/agones-sdk:0.9.0-7d53bcc
• image: gcr.io/agones-images/agones-ping:0.9.0-7d53bcc
• Linux C++ SDK (build: dev): agonessdk-0.9.0-7d53bcc-dev-linux-arch_64.tar.gz
• Linux C++ SDK (build: runtime): agonessdk-0.9.0-7d53bcc-runtime-linux-arch_64.tar.gz
• Linux C++ SDK (source): agonessdk-0.9.0-7d53bcc-src.zip
• SDK Server: agonessdk-server-0.9.0-7d53bcc.zip

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

• https://7d53bcc-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/671/head:pr_671 && git checkout pr_671
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.9.0-7d53bcc

[markmandel]

🐇