ACPI: thinkpad-acpi: trivial fix to documentation

Fix a stray ibm-acpi that should have been replaced with thinkpad-acpi.
Thanks to Damjan <gdamjan@mail.net.mk> for noticing this one.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>