Update badge in README.md (#384)

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Author: kahaaga

README.md

@@ -3,7 +3,7 @@
 [![CI](https://github.com/juliadynamics/Associations.jl/workflows/CI/badge.svg)](https://github.com/JuliaDynamics/Associations.jl/actions)
 [![](https://img.shields.io/badge/docs-latest_tagged-blue.svg)](https://juliadynamics.github.io/Associations.jl/stable/)
 [![](https://img.shields.io/badge/docs-dev_(main)-blue.svg)](https://juliadynamics.github.io/Associations.jl/dev/)
-[[![codecov](https://codecov.io/gh/JuliaDynamics/Associations.jl/branch/master/graph/badge.svg?token=0b71n6x6AP)](https://codecov.io/gh/JuliaDynamics/Associations.jl)](https://img.shields.io/codecov/c/github/juliadynamics/Associations.jl/main)
+[![codecov](https://codecov.io/gh/JuliaDynamics/Associations.jl/branch/main/graph/badge.svg?token=0b71n6x6AP)](https://codecov.io/gh/JuliaDynamics/Associations.jl)
 [![DOI](https://zenodo.org/badge/135443027.svg)](https://zenodo.org/badge/latestdoi/135443027)
 
 Associations.jl is a package for quantifying associations, independence testing and causal inference.
@@ -33,4 +33,4 @@ we also offer
 
 To install the package, run `import Pkg; Pkg.add("Associations")`.
 
-*Previously, this package was called CausalityTools.jl*.
+*Previously, this package was called CausalityTools.jl*.