Use CI status badge from main instead of any PR (#100)

Signed-off-by: HARPER Jon <[email protected]>

Author: jonenst

README.md

@@ -1,5 +1,5 @@
 # Study Notification Server
 
-[![Actions Status](https://github.com/gridsuite/study-notification-server/workflows/CI/badge.svg)](https://github.com/gridsuite/study-notification-server/actions)
+[![Actions Status](https://github.com/gridsuite/study-notification-server/actions/workflows/build.yml/badge.svg?branch=main)](https://github.com/gridsuite/study-notification-server/actions)
 [![Coverage Status](https://sonarcloud.io/api/project_badges/measure?project=org.gridsuite%3Astudy-notification-server&metric=coverage)](https://sonarcloud.io/component_measures?id=org.gridsuite%3Astudy-notification-server&metric=coverage)
 [![MPL-2.0 License](https://img.shields.io/badge/license-MPL_2.0-blue.svg)](https://www.mozilla.org/en-US/MPL/2.0/)