fix(docs): update the non-existent main.yml badge (testcontainers#493)

The .github/workflows/main.yml has already been removed. This caused the
badge on the tutorial page not to be displayed, so it has been updated in index.rst (to ci-core.yml)


![image](https://github.com/testcontainers/testcontainers-python/assets/30658134/f6df32ce-aa93-43f5-becc-2dd8ec0322d7)

# Target Page
https://testcontainers-python.readthedocs.io/en/latest/README.html

---------

Co-authored-by: Dave Ankin <[email protected]>

Author: 2 people

index.rst

@@ -1,8 +1,8 @@
 testcontainers-python
 =====================
 
-.. image:: https://github.com/testcontainers/testcontainers-python/workflows/testcontainers-python/badge.svg
-   :target: https://github.com/testcontainers/testcontainers-python/actions/workflows/main.yml
+.. image:: https://github.com/testcontainers/testcontainers-python/actions/workflows/ci-core.yml/badge.svg
+   :target: https://github.com/testcontainers/testcontainers-python/actions/workflows/ci-core.yml
 .. image:: https://img.shields.io/pypi/v/testcontainers.svg
    :target: https://pypi.python.org/pypi/testcontainers
 .. image:: https://readthedocs.org/projects/testcontainers-python/badge/?version=latest