Update Github Action badge to reference the CI status page. (#549)

* Update Github Action badge.

* Update badge name.

README.md

-![](https://github.com/google/pprof/workflows/ci/badge.svg)
-[![Build Status](https://travis-ci.org/google/pprof.svg?branch=master)](https://travis-ci.org/google/pprof)
-[![codecov](https://codecov.io/gh/google/pprof/graph/badge.svg)](https://codecov.io/gh/google/pprof)
+[![Github Action CI](https://github.com/google/pprof/workflows/ci/badge.svg)](https://github.com/google/pprof/actions)
+[![Travis CI](https://travis-ci.org/google/pprof.svg?branch=master)](https://travis-ci.org/google/pprof)
+[![Codecov](https://codecov.io/gh/google/pprof/graph/badge.svg)](https://codecov.io/gh/google/pprof)
 
 # Introduction