fixed mininet start up cmd perse error
fixed mininet start up cmd perse error

muzixing/ryu,lzppp/mylearning,muzixing/ryu,lzppp/mylearning,muzixing/ryu,muzixing/ryu,muzixing/ryu,lzppp/mylearning,lzppp/mylearning,lzppp/mylearning