Merge #23: Fix example spend amount

49ec983 Fix example spend amount (Tobin C. Harding)

Pull request description:

  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](https://github.com/rust-bitcoin/rust-bitcoin).

ACKs for top commit:
  apoelstra:
    ACK 49ec983

Tree-SHA512: e7487ca04ab579b044e36a6a171c12a5f27872d7aaf95f0114d18872226d2f74174bdb7dd03072c4feecf9c25bb99fc97cdd20c0d76515c51413b95b2a8e7ce8

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).