update go report url to match module name (#664)

Go report shows report for the latest version associated with the
relevant module (name), hence the major version.

README.md

@@ -1,4 +1,4 @@
-[![Go Report](https://goreportcard.com/badge/github.com/peak/s5cmd)](https://goreportcard.com/report/github.com/peak/s5cmd) ![Github Actions Status](https://github.com/peak/s5cmd/actions/workflows/ci.yml/badge.svg)
+[![Go Report](https://goreportcard.com/badge/github.com/peak/s5cmd/v2)](https://goreportcard.com/report/github.com/peak/s5cmd/v2) ![Github Actions Status](https://github.com/peak/s5cmd/actions/workflows/ci.yml/badge.svg)
 
 ![](./doc/s5cmd_header.jpg)