[skip-changelog] chore: fix codecov badge url (#2022)

README.md

@@ -8,7 +8,7 @@ and many other tools needed to use any Arduino compatible board and platform fro
 [![Test Go status](https://github.com/arduino/arduino-cli/actions/workflows/test-go-task.yml/badge.svg)](https://github.com/arduino/arduino-cli/actions/workflows/test-go-task.yml)
 [![Publish Nightly Build status](https://github.com/arduino/arduino-cli/actions/workflows/publish-go-nightly-task.yml/badge.svg)](https://github.com/arduino/arduino-cli/actions/workflows/publish-go-nightly-task.yml)
 [![Deploy Website status](https://github.com/arduino/arduino-cli/actions/workflows/deploy-cobra-mkdocs-versioned-poetry.yml/badge.svg)](https://github.com/arduino/arduino-cli/actions/workflows/deploy-cobra-mkdocs-versioned-poetry.yml)
-[![Codecov](https://codecov.io/gh/arduino/arduino-cli/branch/main/graph/badge.svg)](https://codecov.io/gh/arduino/arduino-cli)
+[![Codecov](https://codecov.io/gh/arduino/arduino-cli/branch/master/graph/badge.svg)](https://codecov.io/gh/arduino/arduino-cli)
 
 > **Note:** this software is currently under active development: anything can change at any time, API and UI must be
 > considered unstable until we release version 1.0.0.