docs(readme): fix upgrade link

Author: AllanZhengYP

README.md

@@ -136,7 +136,7 @@ It returns a function that accepts args, an object that contains the parameters
 #### Other Changes
 
 If you are looking for a breakdown of the API changes from AWS SDK for JavaScript v2 to v3,
-we have them listed in [UPGRADING.md](./UPGRADING.md).
+we have them listed in [UPGRADING.md](https://github.com/aws/aws-sdk-js-v3/blob/main/UPGRADING.md).
 
 ### Install from Source