Fix codecov badge (#403)

* Fix codecov badge

* Update README.md

* Update README.md

* Update README.md

Author: Tortar

README.md

@@ -3,7 +3,7 @@
 [![Documentation stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliagraphs.org/Graphs.jl/stable/)
 [![Documentation dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliagraphs.org/Graphs.jl/dev/)
 [![Build status](https://github.com/JuliaGraphs/Graphs.jl/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/JuliaGraphs/Graphs.jl/actions/workflows/ci.yml?query=branch%3Amaster)
-[![Code coverage](http://codecov.io/github/JuliaGraphs/Graphs.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaGraphs/Graphs.jl?branch=master)
+[![Code coverage](https://codecov.io/github/JuliaGraphs/Graphs.jl/coverage.svg?branch=master)](https://codecov.io/github/JuliaGraphs/Graphs.jl?branch=master)
 [![Code style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/invenia/BlueStyle)
 [![Aqua QA](https://raw.githubusercontent.com/JuliaTesting/Aqua.jl/master/badge.svg)](https://github.com/JuliaTesting/Aqua.jl)
 [![ColPrac: Contributor's Guide on Collaborative Practices for Community Packages](https://img.shields.io/badge/ColPrac-Contributor's%20Guide-blueviolet)](https://github.com/SciML/ColPrac)