Fix game loop CPU API references

[junaga]

What changed

Update the game-loop article to use the current Game.cpu.limit property and Game.cpu.getUsed method.

Why

The article still referenced the removed Game.cpuLimit and Game.getUsedCpu APIs, producing two broken links and directing players to obsolete names.

Validation

• Confirmed both target declarations in api/source/Cpu.md
• Generated the docs with the repository's Node 10 build environment
• git diff --check