updating sign transaction function

[Husienvora]

Updated SignTransaction function to

async signTransaction(rawTx, privateKey) {
    const tx = new Tx(rawTx);

    const pkey = Buffer.from(privateKey, "hex");

    tx.sign(pkey);

    const signedTx = `0x${tx.serialize().toString("hex")}`;

    return signedTx;
  }