Modify copyright header on Bech32 code

src/bech32.cpp

@@ -1,4 +1,5 @@
 // Copyright (c) 2017, 2021 Pieter Wuille
+// Copyright (c) 2021 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.