Update coveralls link

Author: jan-janssen

README.md

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 # pylammpsmpi - Parallel Lammps Python interface
-[![Coverage Status](https://coveralls.io/repos/github/pyiron/pylammpsmpi/badge.svg)](https://coveralls.io/github/pyiron/pylammpsmpi)
+[![Coverage Status](https://coveralls.io/repos/github/pyiron/pylammpsmpi/badge.svg?branch=main)](https://coveralls.io/github/pyiron/pylammpsmpi?branch=main)
 [![Python package](https://github.com/pyiron/pylammpsmpi/workflows/Python%20package/badge.svg)](https://github.com/pyiron/pylammpsmpi/actions)
 
 With `pylammpsmpi` you can control a `mpi4py` parallel LAMMPS instance from a serial python process or a Jupyter