make erc721 cairo link version-agnostic

Author: 0xNeshi

pages/applications/erc721.md

@@ -20,4 +20,4 @@ Here's an implementation of the ERC721 interface in Cairo:
 // [!include ~/listings/applications/erc721/src/erc721.cairo]
 ```
 
-There are other implementations, such as the [Open Zeppelin](https://docs.openzeppelin.com/contracts-cairo/0.20.0/erc721) one.
+There are other implementations, such as the [Open Zeppelin](https://docs.openzeppelin.com/contracts-cairo/erc721) one.