ACPI: thinkpad-acpi: use uppercase for "LED" on user documentation

Change all occourences of the "led" word to full uppercase in user
documentation.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Len Brown <len.brown@intel.com>