Update badge URL (#21)

The previous URL showed the status of the latest run of the action
(e.g., on a pull request). The URL that we switch to in this patch
displays the status of the latest run for the default branch [1]:

> By default, badges display the status of your default branch. If there
> are no workflow runs on your default branch, it will display the
> status of the most recent run across all branches.

[1]: https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/monitoring-workflows/adding-a-workflow-status-badge

README.md

@@ -1,6 +1,4 @@
-# Cleanlab Codex [![Build Status](https://github.com/cleanlab/cleanlab-codex/workflows/CI/badge.svg)](https://github.com/cleanlab/cleanlab-codex/actions?query=workflow%3ACI) [![PyPI - Version](https://img.shields.io/pypi/v/cleanlab-codex.svg)](https://pypi.org/project/cleanlab-codex) [![PyPI - Python Version](https://img.shields.io/pypi/pyversions/cleanlab-codex.svg)](https://pypi.org/project/cleanlab-codex)
-
------
+# Cleanlab Codex [![Build Status](https://github.com/cleanlab/cleanlab-codex/actions/workflows/ci.yml/badge.svg)](https://github.com/cleanlab/cleanlab-codex/actions/workflows/ci.yml) [![PyPI - Version](https://img.shields.io/pypi/v/cleanlab-codex.svg)](https://pypi.org/project/cleanlab-codex) [![PyPI - Python Version](https://img.shields.io/pypi/pyversions/cleanlab-codex.svg)](https://pypi.org/project/cleanlab-codex)
 
 ## Table of Contents