fix badge URL to track github change

Author: petercorke

README.md

@@ -10,7 +10,7 @@
 [![Powered by Open3D](https://raw.githubusercontent.com/petercorke/machinevision-toolbox-python/master/.github/svg/open3d_powered.svg)](https://open3d.org)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
-[![Build Status](https://github.com/petercorke/machinevision-toolbox-python/workflows/Test-master/badge.svg?branch=master)](https://github.com/petercorke/machinevision-toolbox-python/actions?query=workflow%3Abuild)
+[![Build Status](https://github.com/petercorke/machinevision-toolbox-python/actions/workflows/main.yml/badge.svg)](https://github.com/petercorke/machinevision-toolbox-python/actions?query=workflow%3Abuild)
 [![Coverage](https://codecov.io/gh/petercorke/machinevision-toolbox-python/branch/master/graph/badge.svg)](https://codecov.io/gh/petercorke/machinevision-toolbox-python)
 [![PyPI - Downloads](https://img.shields.io/pypi/dw/machinevision-toolbox-python)](https://pypistats.org/packages/machinevision-toolbox-python)