Fix base64 dependency path in README.md examples

Author: notmandatory

README.md

@@ -95,7 +95,7 @@ use bdk::blockchain::ElectrumBlockchain;
 use bdk::electrum_client::Client;
 use bdk::wallet::AddressIndex::New;
 
-use bitcoin::base64;
+use base64;
 use bitcoin::consensus::serialize;
 
 fn main() -> Result<(), bdk::Error> {
@@ -132,7 +132,7 @@ fn main() -> Result<(), bdk::Error> {
 ```rust,no_run
 use bdk::{Wallet, SignOptions, database::MemoryDatabase};
 
-use bitcoin::base64;
+use base64;
 use bitcoin::consensus::deserialize;
 
 fn main() -> Result<(), bdk::Error> {