The section on "Mutating Source Files in Place" in the Zig Build System guide is outdated

[lishaduck]

As of ziglang/zig#20580 / ziglang/zig@d1c14f2, ziglang/zig#14944 is done (issue still needs to be marked as completed), the example at content/en-US/learn/build-system.smd:413 needs updating.