docs: Update CI status badge to point to GitHub Actions

Author: bd808

doc/index.rst

@@ -6,8 +6,8 @@ IRC bot framework using mixins to provide commonly desired functionality.
 .. image:: https://img.shields.io/pypi/v/ib3.svg
    :target: https://pypi.org/project/ib3
 
-.. image:: https://img.shields.io/travis/bd808/python-ib3/master.svg
-   :target: http://travis-ci.org/bd808/python-ib3
+.. image:: https://img.shields.io/github/actions/workflow/status/bd808/python-ib3/tox.yml?logo=github&style=for-the-badge
+   :target: https://github.com/bd808/python-ib3/actions/workflows/tox.yml
 
 .. image:: https://readthedocs.org/projects/python-ib3/badge/?version=latest
    :target: https://python-ib3.readthedocs.io