null_blk: corrections to documentation

Randy Dunlap reported a couple of grammar errors and unfortunate usages of
socket/node/core.

Signed-off-by: Matias Bjorling <m@bjorling.me>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>