[docs][README] Fix RTD sticker (#147)

**Why this change was necessary**
RTD sticker in README was not pointing to the RTD project, making it
show "Unknown" for the RTD build status.

**What this change does**
Sticker points to
https://readthedocs.org/projects/shimming-toolbox-py/ , but links to
https://shimming-toolbox.org/en/latest/

**Any side-effects?**
None.

README.md

@@ -1,6 +1,6 @@
 # shimming-toolbox
 
- [![Build Status](https://github.com/shimming-toolbox/shimming-toolbox/workflows/CI-Tests/badge.svg)](https://github.com/shimming-toolbox/shimming-toolbox/actions?query=workflow%3ACI-Tests) [![Documentation Status](https://readthedocs.org/projects/shimming-toolbox/badge/?version=latest)](https://www.shimming-toolbox.org/en/latest/?badge=latest) [![Coverage Status](https://coveralls.io/repos/github/shimming-toolbox/shimming-toolbox/badge.svg?branch=master)](https://coveralls.io/github/shimming-toolbox/shimming-toolbox?branch=master)
+ [![Build Status](https://github.com/shimming-toolbox/shimming-toolbox/workflows/CI-Tests/badge.svg?branch=master)](https://github.com/shimming-toolbox/shimming-toolbox/actions?query=workflow%3ACI-Tests+branch%3Amaster) [![Coverage Status](https://coveralls.io/repos/github/shimming-toolbox/shimming-toolbox/badge.svg?branch=master)](https://coveralls.io/github/shimming-toolbox/shimming-toolbox?branch=master) [![Documentation Status](https://readthedocs.org/projects/shimming-toolbox-py/badge/?version=latest)](https://shimming-toolbox.org/en/latest/)
 
 
 Code for performing real-time shimming using external MRI shim coils