Fix bttc, maticpoly, matic

bttc mapped to trx, so changed the label instead of changing the mapping

maticpoly was mislabeled on changenow

maticpoly and matic were mislabeled on simpleswap

lib/exchange/changenow/changenow_exchange_provider.dart

@@ -273,6 +273,8 @@ class ChangeNowExchangeProvider extends ExchangeProvider {
         return 'zec';
       case CryptoCurrency.usdcpoly:
         return 'usdcmatic';
+      case CryptoCurrency.maticpoly:
+        return 'maticmainnet';
       default:
         return currency.title.toLowerCase();
     }

lib/exchange/simpleswap/simpleswap_exchange_provider.dart

@@ -231,6 +231,10 @@ class SimpleSwapExchangeProvider extends ExchangeProvider {
         return 'usdcpoly';
       case CryptoCurrency.usdcsol:
         return 'usdcspl';
+      case CryptoCurrency.matic:
+        return 'maticerc20';
+      case CryptoCurrency.maticpoly:
+        return 'matic';
       default:
         return currency.title.toLowerCase();
     }