docs: update contract id info

Fixes blockstack/docs#1009

src/pages/references/stacking-contract.md

@@ -8,7 +8,7 @@ import { StackingErrorcodeReference, StackingFunctionReference } from '@componen
 
 ## Introduction
 
-Stacking is implemented as a smart contract using Clarity. On the testnet, you can always find the Stacking contract by this contract identifier: `ST000000000000000000002AMW42H.pox`.
+Stacking is implemented as a smart contract using Clarity. You can always find the Stacking contract identifier using the Stacks Blockchain API [`v2/pox` endpoint](https://blockstack.github.io/stacks-blockchain-api/#operation/get_pox_info).
 
 Below is a list of public and read-only functions as well as error codes that can be returned by those methods.