Update NewScript example after API change.

This change updates the doc.go documentation file with the correct use
of the new (since 8eead5217d) API used
for passing additional options when creating new script engines.

Spotted by davec@

doc.go

@@ -44,8 +44,12 @@ engine to validate a transaction.
 	func ValidateTx(tx *btcwire.MsgTx, txIdx int, txIn *btcwire.MsgTx, txInIdx int, timestamp time.Time) {
 		pkScript := txIn.TxOut[txInIdx].PkScript
 		sigScript := tx.txIn[TxIdx]
+		var flags btcscript.ScriptFlags
+		if timestamp.After(btcscript.Bip16Activation) {
+			flags |= btcscript.ScriptBip16
+		}
 		engine, err := btcscript.NewScript(sigScript, pkScript, txInIdx,
-			tx, timestamp.After(btcscript.Bip16Activation))
+			tx, flags)
 		return engine.Execute()
 	}