Remove one JSON related MUST requirement
Remove one JSON related MUST requirement

JSON is not a mandatory format, we should not use MUST
requirement on this.

Signed-off-by: Qiang Huang <488e06933c295c76472a67dbc6657ac40a756953@huawei.com>

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