config: fix minor issues around grammar and formatting
config: fix minor issues around grammar and formatting

Various markdown, and grammar issues summarized here: https://github.com/opencontainers/image-spec/pull/333#issuecomment-249000454

Thank you to wking and coolljt0725 for the detailed review.

Signed-off-by: Brandon Philips <74d2b51265fa19d338eb593a7555ec2b932e59c0@coreos.com>

jonboulle/image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,opencontainers/image-spec,vbatts/oci-image-spec,opencontainers/image-spec