readme: use shields.io for codecov badge

README.md

@@ -1,6 +1,6 @@
 [![Master status](https://tc.gortc.io/app/rest/builds/buildType:(id:stun_MasterStatus)/statusIcon.svg)](https://tc.gortc.io/project.html?projectId=stun&tab=projectOverview&guest=1)
 [![GoDoc](https://godoc.org/gortc.io/stun?status.svg)](http://godoc.org/gortc.io/stun)
-[![codecov](https://codecov.io/gh/gortc/stun/branch/master/graph/badge.svg)](https://codecov.io/gh/gortc/stun)
+[![codecov](https://img.shields.io/codecov/c/github/gortc/stun?label=coverage)](https://codecov.io/gh/gortc/stun)
 # STUN
 Package stun implements Session Traversal Utilities for NAT (STUN) [[RFC5389](https://tools.ietf.org/html/rfc5389)]
 protocol and [client](https://godoc.org/gortc.io/stun#Client) with no external dependencies and zero allocations in hot paths.