Update README.md: add codecov badge

README.md

@@ -2,7 +2,7 @@
 
 [![pkgeval](https://juliahub.com/docs/StructIO/pkgeval.svg)](https://juliahub.com/ui/Packages/StructIO/aDnIf)
 [![Build Status](https://github.com/JuliaIO/StructIO.jl/workflows/CI/badge.svg)](https://github.com/JuliaIO/StructIO.jl/actions?query=workflow%3ACI+branch%3Amaster)
-[![Coverage](http://codecov.io/github/JuliaIO/StructIO.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaIO/StructIO.jl?branch=master)
+[![Coverage](https://codecov.io/gh/JuliaIO/StructIO.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaIO/StructIO.jl)
 
 Generates IO methods (`pack`, `unpack`) from structure definitions. Also defines `packed_sizeof` to give the on-disk size of a packed structure, which is smaller than `sizeof` would give, if the struct is marked as `align_packed`.