Use RFC_2119 style for runtime_linux.md
Use RFC_2119 style for runtime_linux.md

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

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