kbuild: fix for some typos in Documentation/makefiles.txt

I noticed a few typos while reading makefiles.txt to learn about the
kbuild system.  Attached is a patch against 2.6.18 to fix them.
Remove trailing whitespace while we are there..
Signed-off-by: Bryce Harrington <bryce@osdl.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>