README: remove extraneous apostrophe
README: remove extraneous apostrophe

Also s/OpenContainers/OCI/ as used in the image-spec README.

Signed-off-by: Jonathan Boulle <0d47477beef1d882e8812885f3dcf66c4a47c346@gmail.com>

laijs/specs,wking/opencontainer-runtime-spec,opencontainers/specs,cloudfoundry-incubator/specs,opencontainers/runtime-spec,hqhq/runtime-spec,jonboulle/specs,ujuc/specs,hqhq/specs,wking/opencontainer-specs