Fix workflow badge visibility in README

co-authored-by: Andrea Blengino <ing.andrea.blengino@protonmail.com>

README.rst

@@ -2,7 +2,7 @@
 django-cors-headers
 ===================
 
-.. image:: https://img.shields.io/github/actions/workflow/status/adamchainz/django-cors-headers/main.yml?branch=main&style=for-the-badge
+.. image:: https://img.shields.io/github/actions/workflow/status/adamchainz/django-cors-headers/main.yml.svg?branch=main&style=for-the-badge
    :target: https://github.com/adamchainz/django-cors-headers/actions?workflow=CI
 
 .. image:: https://img.shields.io/badge/Coverage-100%25-success?style=for-the-badge