fix xplat release notes readme for new markdown parser.
fix xplat release notes readme for new markdown parser.

ChrisLGardner/vNextBuild,rfennell/vNextBuild,rfennell/vNextBuild,rfennell/vNextBuild,ChrisLGardner/vNextBuild,rfennell/vNextBuild,ChrisLGardner/vNextBuild,ChrisLGardner/vNextBuild