Update steam.mdx: Set build step id to `buildStep` to help readers avoid confusion.

[davidmfinol]

Changes

• Set build step id to buildStep to help readers avoid confusion.

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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Visit the preview URL for this PR (updated for commit d17d1de):

https://game-ci-5559f--pr466-davidmfinol-patch-4-nfzajujk.web.app

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