Fix Sample for OEM Device Setup

[mburumaxwell]

Updated oem-device-setup.md to fix the following errors:

• Use files instead of Files, file instead of File, destinationDir instead of DestinationDir
• destinationDir can only start with predefined runtime macros e.g. $(runtime.bootDrive)
• The DMSetup.cmd script must have a destination directory to allow execution from oemcustomization.cmd hence use the OEMInstall folder
• Limpet.exe is not optional and should be installed to system32 folder

References: [Windows Universal OEM Package Schema] (https://docs.microsoft.com/en-us/windows-hardware/manufacture/iot/package-schema)

[msftclas]

All CLA requirements met.

[gmileka]

Thank you!