config.md: typo and sentence fix
config.md: typo and sentence fix

Signed-off-by: Ma Shimiao <4511f16e942ad3568742865d09c9060c1d8bebb9@cn.fujitsu.com>

jonboulle/image-spec,opencontainers/image-spec,jonboulle/image-spec,opencontainers/image-spec