docs: add usage clarification to File builder

[RamsaiPolisetti]

This PR adds a small clarification to the File builder documentation.

The File builder requires that the source path provided by the user
exists on the local machine running Packer. When the path is incorrect,
Packer will raise a "file not found" error during the build stage.

This brief note helps users avoid a common misconfiguration and improves
the overall clarity of the File builder usage instructions.

Docs-only change. No functional impact.

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