runtime.md: fix spacing
runtime.md: fix spacing

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

cloudfoundry-incubator/specs,hqhq/specs,wking/opencontainer-runtime-spec,marcosnils/specs,gao-feng/specs,opencontainers/runtime-spec,hqhq/runtime-spec,jonboulle/specs,vbatts/specs,wangkirin/specs,ujuc/specs,opencontainers/specs,ChengTiesheng/specs,laijs/specs,runcom/specs,wking/opencontainer-specs