README.md: remove /mod from badge URL

Change-Id: I856048c70041287c7f71d26c20b4c565b3abe61e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/276052
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/README.md b/README.md
index 58892f2..fff9012 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # Go support for Protocol Buffers
 
-[![Go Reference](https://pkg.go.dev/badge/google.golang.org/protobuf.svg)](https://pkg.go.dev/mod/google.golang.org/protobuf)
+[![Go Reference](https://pkg.go.dev/badge/google.golang.org/protobuf.svg)](https://pkg.go.dev/google.golang.org/protobuf)
 [![Build Status](https://travis-ci.org/protocolbuffers/protobuf-go.svg?branch=master)](https://travis-ci.org/protocolbuffers/protobuf-go)
 
 This project hosts the Go implementation for