Update build status badge

At some point the CI yaml file was renamed and the badge is generated
by referring to yaml file name, so it needs to be updated.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

README.md

@@ -1,6 +1,6 @@
 # Frequenz Python SDK
 
-[![Build Status](https://github.com/frequenz-floss/frequenz-sdk-python/actions/workflows/ci.yaml/badge.svg)](https://github.com/frequenz-floss/frequenz-sdk-python/actions/workflows/ci.yaml)
+[![Build Status](https://github.com/frequenz-floss/frequenz-sdk-python/actions/workflows/ci-push.yaml/badge.svg)](https://github.com/frequenz-floss/frequenz-sdk-python/actions/workflows/ci-push.yaml)
 [![PyPI Package](https://img.shields.io/pypi/v/frequenz-sdk)](https://pypi.org/project/frequenz-sdk/)
 [![Docs](https://img.shields.io/badge/docs-latest-informational)](https://frequenz-floss.github.io/frequenz-sdk-python/)