Documentation: thinkpad-acpi grammo fixes

This adds minor grammatical fixes to the description of the keys in the
thinkpad-acpi documentation.

Signed-off-by: Kumar Appaiah <a.kumar@alumni.iitm.ac.in>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>