clippy: fix the ignored_unit_patterns lint

Just style.

Author: apoelstra

examples/tx.rs

@@ -32,7 +32,7 @@ fn main() {
 
     let res = tx.verify_tx_amt_proofs(&secp, &[txout]);
     match res {
-        Ok(_) => {}
+        Ok(()) => {}
         Err(e) => {
             panic!("{}", e);
         }