Fix ScanForWalletTransactions start_block comment
Fix ScanForWalletTransactions start_block comment

Suggested https://github.com/bitcoin/bitcoin/pull/14711#discussion_r252036436

chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin