Documentation: update kbuild make typos/grammar/text flow

Update section 3.7 examples to reflect the current state of the
Makefiles used. Fix spelling and grammar errors along with flow of text.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>