"examples/4_Event_Detectors.ipynb" did not exist on "9de44861429763da1a652e754d37fcc92ba3f1d5"

Rename `master` branch to `main` to follow Orekit naming conventions

Orekit, Orekit Tutorials, and Orekit Data projects recently renamed their master branch to main.
It would be nice to do the same in OreCzml to keep the same naming conventions.
It can wait for the next minor or major version.