Fix example spend amount

In the segwit signing example we are using the incorrect value when
creating the signature - we should be using the utxo amount (input
amount) not the spend amount (output spend amount).

Also fixed in `rust-bitcoin` PR #2696.

Author: tcharding

cookbook/src/tx_segwit-v0.md

@@ -204,7 +204,7 @@ fn main() {
     let sighash_type = EcdsaSighashType::All;
     let mut sighasher = SighashCache::new(&mut unsigned_tx);
     let sighash = sighasher
-        .p2wpkh_signature_hash(input_index, &dummy_utxo.script_pubkey, SPEND_AMOUNT, sighash_type)
+        .p2wpkh_signature_hash(input_index, &dummy_utxo.script_pubkey, DUMMY_UTXO_AMOUNT, sighash_type)
         .expect("failed to create sighash");
 
     // Sign the sighash using the secp256k1 library (exported by rust-bitcoin).