chore(readme): update codecov badge

Author: shuheng-liu

README.md

@@ -1,7 +1,7 @@
 # neurodiffeq
 
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/eada52ca726e4919923e213b81ee6420)](https://app.codacy.com/gh/odegym/neurodiffeq?utm_source=github.com&utm_medium=referral&utm_content=odegym/neurodiffeq&utm_campaign=Badge_Grade_Settings)
-![PyPI](https://img.shields.io/pypi/v/neurodiffeq?color=blueviolet&label=PyPI&logoColor=blueviolet) ![GitHub issues](https://img.shields.io/github/issues/NeuroDiffGym/neurodiffeq?color=green) [![Build Status](https://travis-ci.org/NeuroDiffGym/neurodiffeq.svg?branch=master)](https://travis-ci.org/NeuroDiffGym/neurodiffeq) [![codecov](https://codecov.io/gh/odegym/neurodiffeq/branch/master/graph/badge.svg)](https://codecov.io/gh/odegym/neurodiffeq) [![Documentation Status](https://readthedocs.org/projects/neurodiffeq/badge/?version=latest)](https://neurodiffeq.readthedocs.io/en/latest/?badge=latest) [![DOI](https://joss.theoj.org/papers/10.21105/joss.01931/status.svg)](https://doi.org/10.21105/joss.01931)
+![PyPI](https://img.shields.io/pypi/v/neurodiffeq?color=blueviolet&label=PyPI&logoColor=blueviolet) ![GitHub issues](https://img.shields.io/github/issues/NeuroDiffGym/neurodiffeq?color=green) [![Build Status](https://travis-ci.org/NeuroDiffGym/neurodiffeq.svg?branch=master)](https://travis-ci.org/NeuroDiffGym/neurodiffeq) [![codecov](https://codecov.io/gh/NeuroDiffGym/neurodiffeq/branch/master/graph/badge.svg)](https://codecov.io/gh/NeuroDiffGym/neurodiffeq) [![Documentation Status](https://readthedocs.org/projects/neurodiffeq/badge/?version=latest)](https://neurodiffeq.readthedocs.io/en/latest/?badge=latest) [![DOI](https://joss.theoj.org/papers/10.21105/joss.01931/status.svg)](https://doi.org/10.21105/joss.01931)
 
 # Introduction