kbuild: linguistic fixes for Documentation/kbuild/makefiles.txt

I have done a look-through through Documentation/kbuild/ and my corrections
(proposed) are attached.

Cc'ed are original author Michael (responsible for comitting changes to
these files?), Sam (kbuild maintainer), Adrian (-trivial maintainer).
Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>