[docs] Fixed CI badge in README

Author: nemesifier

README.rst

@@ -1,8 +1,8 @@
 netjsonconfig
 =============
 
-.. image:: https://github.com/openwisp/netjsonconfig/workflows/Netjsonconfig%20CI%20Build/badge.svg?branch=master
-    :target: https://github.com/openwisp/netjsonconfig/actions?query=workflow%3A%22Netjsonconfig+CI+Build%22
+.. image:: https://github.com/openwisp/netjsonconfig/actions/workflows/ci.yml/badge.svg
+    :target: https://github.com/openwisp/netjsonconfig/actions/workflows/ci.yml
 
 .. image:: https://coveralls.io/repos/openwisp/netjsonconfig/badge.svg
     :target: https://coveralls.io/r/openwisp/netjsonconfig