remove azure pipelines link from README (#771)

Update build badge

Signed-off-by: Fabio Vera <fabiovera@microsoft.com>
Signed-off-by: Keith Battocchi <kebatt@microsoft.com>
Co-authored-by: Keith Battocchi <kebatt@microsoft.com>

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://dev.azure.com/ms/EconML/_apis/build/status/Microsoft.EconML?branchName=main)](https://dev.azure.com/ms/EconML/_build/latest?definitionId=49&branchName=main)
+[![Build status](https://github.com/py-why/EconML/actions/workflows/ci.yml/badge.svg)](https://github.com/py-why/EconML/actions/workflows/ci.yml)
 [![PyPI version](https://img.shields.io/pypi/v/econml.svg)](https://pypi.org/project/econml/)
 [![PyPI wheel](https://img.shields.io/pypi/wheel/econml.svg)](https://pypi.org/project/econml/)
 [![Supported Python versions](https://img.shields.io/pypi/pyversions/econml.svg)](https://pypi.org/project/econml/)