[skip ci] fix coveralls link in README
[skip ci] fix coveralls link in README

denverwilliams/linkerd,denverwilliams/linkerd,BuoyantIO/linkerd,linkerd/linkerd,pawelprazak/linkerd,BuoyantIO/linkerd,hhtpcd/linkerd,linkerd/linkerd,pawelprazak/linkerd,denverwilliams/linkerd,denverwilliams/linkerd,hhtpcd/linkerd,hhtpcd/linkerd,pawelprazak/linkerd,linkerd/linkerd