Fix README badges to use new default 'main' branch

Author: kelson42

README.md

@@ -6,11 +6,11 @@ interface on top of the [C++ `libzim` library](https://github.com/openzim/libzim
 
 It is primarily used in [openZIM](https://github.com/openzim/) scrapers like [`sotoki`](https://github.com/openzim/sotoki) or [`youtube2zim`](https://github.com/openzim/youtube).
 
-[![Build Status](https://github.com/openzim/python-libzim/workflows/test/badge.svg?query=branch%3Amaster)](https://github.com/openzim/python-libzim/actions?query=branch%3Amaster)
+[![Build Status](https://github.com/openzim/python-libzim/workflows/test/badge.svg?query=branch%3Amain)](https://github.com/openzim/python-libzim/actions?query=branch%3Amain)
 [![CodeFactor](https://www.codefactor.io/repository/github/openzim/python-libzim/badge)](https://www.codefactor.io/repository/github/openzim/python-libzim)
 [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 [![PyPI version shields.io](https://img.shields.io/pypi/v/libzim.svg)](https://pypi.org/project/libzim/)
-[![codecov](https://codecov.io/gh/openzim/python-libzim/branch/master/graph/badge.svg)](https://codecov.io/gh/openzim/python-libzim)
+[![codecov](https://codecov.io/gh/openzim/python-libzim/branch/main/graph/badge.svg)](https://codecov.io/gh/openzim/python-libzim)
 
 ## Installation