added symbol mapping in cancelOrder
added symbol mapping in cancelOrder

1. unlike other exchanges, Bcex requires to provide symbol in cancelOrder();
2. bcex formats symbol as  "eth2usdt" while ccxt uses the convention of "ETH/USDT". A mapping is needed in cancelOrder();
ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt