replace must with MUST
replace must with MUST

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

opencontainers/specs,wking/opencontainer-specs,hqhq/specs,wking/opencontainer-runtime-spec,opencontainers/runtime-spec,laijs/specs,hqhq/runtime-spec