Fix links.

Author: shyuep

README.md

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 [![CI Status](https://github.com/materialsproject/pymatgen/actions/workflows/test.yml/badge.svg)](https://github.com/materialsproject/pymatgen/actions/workflows/test.yml)
 [![Coveralls](https://img.shields.io/coveralls/github/materialsproject/pymatgen?logo=coveralls&label=Coverage)](https://coveralls.io/github/materialsproject/pymatgen?branch=master)
 [![PyPI Downloads](https://img.shields.io/pypi/dm/pymatgen?logo=pypi&logoColor=white&color=blue&label=PyPI)](https://pypi.org/project/pymatgen)
 [![Conda Downloads](https://img.shields.io/conda/dn/conda-forge/pymatgen?logo=condaforge&color=blue&label=Conda)](https://anaconda.org/conda-forge/pymatgen)
 [![Requires Python 3.8+](https://img.shields.io/badge/Python-3.8+-blue.svg?logo=python&logoColor=white)](https://python.org/downloads)
 [![arXiv](https://img.shields.io/badge/J.ComMatSci-2012.10.028-blue)](https://doi.org/10.1016/j.commatsci.2012.10.028)
 
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 Pymatgen (Python Materials Genomics) is a robust, open-source Python
 library for materials analysis. These are some of the main features: