Update block explorer in banano.dart

Replace BananoLooker (down) with official Creeper instance run by Banano team.

(cherry picked from commit f5432ab)

Author: Verycutecat

lib/wallets/crypto_currency/coins/banano.dart

@@ -89,7 +89,7 @@ class Banano extends NanoCurrency {
   Uri defaultBlockExplorer(String txid) {
     switch (network) {
       case CryptoCurrencyNetwork.main:
-        return Uri.parse("https://www.bananolooker.com/block/$txid");
+        return Uri.parse("https://creeper.banano.cc/hash/$txid");
       default:
         throw Exception(
           "Unsupported network for defaultBlockExplorer(): $network",