Update accounts.go
Update accounts.go

`json:"marginAvailable"` => `json:"marginAvail"`
according to Oanda REST API documentation
{
  "accountId" : 8954947,
  "accountName" : "Primary",
  "balance" : 100000,
  "unrealizedPl" : 0,
  "realizedPl" : 0,
  "marginUsed" : 0,
  "marginAvail" : 100000,
  "openTrades" : 0,
  "openOrders" : 0,
  "marginRate" : 0.05,
  "accountCurrency" : "USD"
}
santegoeds/oanda