fix references to master

Author: oxinabox

README.md

@@ -1,8 +1,8 @@
 # FiniteDifferences.jl: Finite Difference Methods
 
-[![CI](https://github.com/JuliaDiff/FiniteDifferences.jl/workflows/CI/badge.svg?branch=master)](https://github.com/JuliaDiff/FiniteDifferences.jl/actions?query=workflow%3ACI)
-[![Build Status](https://travis-ci.org/JuliaDiff/FiniteDifferences.jl.svg?branch=master)](https://travis-ci.org/JuliaDiff/FiniteDifferences.jl)
-[![codecov.io](https://codecov.io/github/JuliaDiff/FiniteDifferences.jl/coverage.svg?branch=master)](https://codecov.io/github/JuliaDiff/FiniteDifferences.jl?branch=master)
+[![CI](https://github.com/JuliaDiff/FiniteDifferences.jl/workflows/CI/badge.svg?branch=main)](https://github.com/JuliaDiff/FiniteDifferences.jl/actions?query=workflow%3ACI)
+[![Build Status](https://travis-ci.org/JuliaDiff/FiniteDifferences.jl.svg?branch=main)](https://travis-ci.org/JuliaDiff/FiniteDifferences.jl)
+[![codecov.io](https://codecov.io/github/JuliaDiff/FiniteDifferences.jl/coverage.svg?branch=main)](https://codecov.io/github/JuliaDiff/FiniteDifferences.jl?branch=main)
 [![PkgEval](https://juliaci.github.io/NanosoldierReports/pkgeval_badges/F/FiniteDifferences.svg)](https://juliaci.github.io/NanosoldierReports/pkgeval_badges/report.html)
 
 [![Latest Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliadiff.org/FiniteDifferences.jl/latest/)