update branch name

README.md

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-[![Build Status](https://github.com/Symmetronic/discrete-wavelets/workflows/build/badge.svg?branch=master)](https://github.com/Symmetronic/discrete-wavelets/actions?query=workflow%3Abuild+branch%3Amaster) [![Coverage Status](https://coveralls.io/repos/github/Symmetronic/discrete-wavelets/badge.svg?branch=master)](https://coveralls.io/github/Symmetronic/discrete-wavelets?branch=master) [![GitHub License](https://img.shields.io/github/license/Symmetronic/discrete-wavelets)](https://github.com/Symmetronic/discrete-wavelets/blob/master/LICENSE) [![NPM Version](https://img.shields.io/npm/v/discrete-wavelets)](https://www.npmjs.com/package/discrete-wavelets) [![Monthly Downloads](https://img.shields.io/npm/dm/discrete-wavelets)](https://npmcharts.com/compare/discrete-wavelets?minimal=true)
+[![Build Status](https://github.com/Symmetronic/discrete-wavelets/workflows/build/badge.svg?branch=main)](https://github.com/Symmetronic/discrete-wavelets/actions?query=workflow%3Abuild+branch%3Amain) [![Coverage Status](https://coveralls.io/repos/github/Symmetronic/discrete-wavelets/badge.svg?branch=main)](https://coveralls.io/github/Symmetronic/discrete-wavelets?branch=main) [![GitHub License](https://img.shields.io/github/license/Symmetronic/discrete-wavelets)](https://github.com/Symmetronic/discrete-wavelets/blob/main/LICENSE) [![NPM Version](https://img.shields.io/npm/v/discrete-wavelets)](https://www.npmjs.com/package/discrete-wavelets) [![Monthly Downloads](https://img.shields.io/npm/dm/discrete-wavelets)](https://npmcharts.com/compare/discrete-wavelets?minimal=true)
 
 # Discrete Wavelets