Update naj-contract.md (#1582)

Update outdated usage of contract api

docs/4.tools/near-api-js/naj-contract.md

@@ -11,9 +11,8 @@ When you instantiate an instance of `Contract` you need to specify the names of
 Then the new instance of `Contract` will have methods with the same names as your smart contract functions.
 For example if you deployed a contract with `my_smart_contract_function` function on it, then this will work:
 ```js
-const contract = new Contract(account, {
+const contract = new Contract(account, "example-contract.testnet", {
   changeMethods: ["my_smart_contract_function"], // your smart-contract has a function `my_smart_contract_function`
-  sender: account
 });
 // `contract` object has `my_smart_contract_function` function on it: 
 contract.my_smart_contract_function()