Documentation/padata.txt: fix typos etc.

Fix typos & grammar.
Use CPU instead of cpu in text.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>