Updated witness command in README.md
Updated witness command in README.md

bitsuperlab/cpp-play2,oxarbitrage/bitshares-core,bigoc/openchain,oxarbitrage/bitshares-core,cryptonomex/graphene,peertracksinc/muse,abitmore/bitshares-2,bigoc/openchain,bitsuperlab/cpp-play2,bigoc/openchain,cryptonomex/graphene,pmconrad/graphene,pmconrad/graphene,abitmore/bitshares-2,abitmore/bitshares-2,bigoc/openchain,cryptonomex/graphene,bitsuperlab/cpp-play2,peertracksinc/muse,bitshares/bitshares-2,bitshares/bitshares-2,oxarbitrage/bitshares-core,peertracksinc/muse,bitshares/bitshares-2,abitmore/bitshares-2,pmconrad/graphene,peertracksinc/muse,bitshares/bitshares-2,oxarbitrage/bitshares-core,pmconrad/graphene