Fix incorrect architecture name for macOS

[Copilot]

This PR corrects an inaccuracy in the cross-compilation documentation for macOS. The documentation incorrectly stated that macOS provides "amd64" toolchains in the default XCode install, when it should be "arm64" instead.

In the macOS context:

• "x64" refers to Intel architecture
• "arm64" refers to Apple Silicon architecture
• "amd64" is not commonly used in macOS terminology (though it's often an alias for x64 in Linux contexts)

This small correction aligns the documentation with Apple's terminology and accurately represents the architectures supported by macOS and XCode.

Fixes #46382.

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[gewarren]

@agocke There's a bug that prevents the Learn build from running on Copilot-created PRs. They've already fixed it, but have to wait to deploy it due to a freeze. If you want to merge this, you'll need to create a new PR yourself from the branch.