unify Exchange.market_close signature to use "name" instead of "coin"

not sure if this worth pursuing but Exchange.market_close uses https://github.com/hyperliquid-dex/hyperliquid-python-sdk/blob/master/hyperliquid/exchange.py#L244 uses "coin" instead of "name" but everywhere else in the code it uses name. I propose to change it to "name" as well