Update custom parameter example for server builds

[danielchipman]

Changes

• Updated GitHub builder documentation example for customParameters usage to reflect modern command for building a server.

Related PR: https://github.com/game-ci/unity-builder/pull/532 Discussion: https://github.com/game-ci/documentation/issues/240

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [x] Tests (added, updated or not needed)

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