Fix typo in URL (#4604)

com.unity.ml-agents/CHANGELOG.md

 #### com.unity.ml-agents (C#)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
  - PyTorch trainers are now the default. See the
- [installation docs](https://github.com/Unity-Technologies/ml-agents/blob/mastere/docs/Installation.md) for
+ [installation docs](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Installation.md) for
  more information on installing PyTorch. For the time being, TensorFlow is still available;
  you can use the TensorFlow backend by adding `--tensorflow` to the CLI, or
  adding `framework: tensorflow` in the configuration YAML. (#4517)