Fix typo in sdk readme which said only two sdks

[reductor]

No description provided.

[googlebot]

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[agones-bot]

Build Succeeded 👏

Build Id: adc1b173-20c6-4066-a153-d7cd40da6d20

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

• image: gcr.io/agones-images/agones-controller:0.5.0-bbf1263
• image: gcr.io/agones-images/agones-sdk:0.5.0-bbf1263
• Linux C++ SDK (build: dev): agonessdk-0.5.0-bbf1263-dev-linux-arch_64.tar.gz
• Linux C++ SDK (build: runtime): agonessdk-0.5.0-bbf1263-runtime-linux-arch_64.tar.gz
• Linux C++ SDK (source): agonessdk-0.5.0-bbf1263-src.zip
• SDK Server: agonessdk-server-0.5.0-bbf1263.zip

(experimental) To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/356/head:pr_356 && git checkout pr_356
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.5.0-bbf1263

[reductor]

I signed it!

[googlebot]

CLAs look good, thanks!

[markmandel]

LGTM!