Fix incorrect link of the CodeCov in README.md
This fix fixes incorrect link of the CodeCov in README.md.
[![Documentation](https://img.shields.io/badge/godoc-reference-blue.svg?style=flat-square)](https://godoc.org/github.com/coredns/coredns)
[![Build Status](https://img.shields.io/travis/coredns/coredns.svg?style=flat-square&label=build)](https://travis-ci.org/coredns/coredns)
-[![Code Coverage](https://img.shields.io/codecov/c/github/coredns/coredns.svg?style=flat-square)](github.com/coredns/coredns)
+[![Code Coverage](https://img.shields.io/codecov/c/github/coredns/coredns/master.svg?style=flat-square)](https://codecov.io/github/coredns/coredns?branch=master)
[![Go Report Card](https://goreportcard.com/badge/github.com/coredns/coredns?style=flat-square)](https://goreportcard.com/report/coredns/coredns)
CoreDNS is a DNS server that started as a fork of [Caddy](https://github.com/mholt/caddy/). It has the
