Switch LICENSE badge in the README to an absolute link (#51)

Following #50, one additional change that was missed was switching the
license link in the README to be absolute instead of relative, as the
latter does not work in the PyPI package description.

Author: kmaziarz

README.md

@@ -8,7 +8,7 @@
 [![CI](https://github.com/microsoft/syntheseus/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/microsoft/syntheseus/actions/workflows/ci.yml)
 [![Python Version](https://img.shields.io/badge/python-3.7+-blue.svg)](https://www.python.org/downloads/)
 [![code style](https://img.shields.io/badge/code%20style-black-202020.svg)](https://github.com/ambv/black)
-[![License](https://img.shields.io/badge/license-MIT-green.svg)](LICENSE)
+[![License](https://img.shields.io/badge/license-MIT-green.svg)](https://github.com/microsoft/syntheseus/blob/main/LICENSE)
 
 Syntheseus is a package for end-to-end retrosynthetic planning.
 - ⚒️ Combines search algorithms and reaction models in a standardized way