message: docs: update readme for sponsored transaction
diff: @@ -193,7 +193,7 @@ const sponsoredTx = await sponsorTransaction(sponsorOptions);
  // for mainnet, use `StacksMainnet()`
  const network = new StacksTestnet();
  -const broadcastResponse = await broadcastTransaction(transaction, network);
  +const broadcastResponse = await broadcastTransaction(sponsoredTx, network);
  const txId = broadcastResponse.txid;
  ```
  