fix a bug in abides-market exchange agent of ABM is fixed in orderbook

[IdiosyncraticDragon]

This bug will make the kernel.run() fails if the user feeds a symbol with its value not equal to "ABM". Like this:

# it is ok for the existing code
symbol = "ABM"
# it will fail and meets KeyError: 'ABM'
symbol = "Code111"
agents.extend([ExchangeAgent(id=0,
                        name="EXCHANGE_AGENT",
                        mkt_open=mkt_open,
                        mkt_close=mkt_close,
                        symbols=[symbol],
                        book_logging=False, #
                        book_log_depth=10,
                        log_orders=True,
                        pipeline_delay=0,
                        computation_delay=0,
                        stream_history=25_000,
                             )
               ]
              )