chore: update badge

Signed-off-by: Frost Ming <[email protected]>

README.md

@@ -5,7 +5,7 @@
 [![Tests](https://github.com/frostming/unearth/workflows/Tests/badge.svg)](https://github.com/frostming/unearth/actions?query=workflow%3Aci)
 [![pypi version](https://img.shields.io/pypi/v/unearth.svg)](https://pypi.org/project/unearth/)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
-[![pdm-managed](https://img.shields.io/badge/pdm-managed-blueviolet)](https://pdm.fming.dev)
+[![pdm-managed](https://img.shields.io/endpoint?url=https%3A%2F%2Fcdn.jsdelivr.net%2Fgh%2Fpdm-project%2F.github%2Fbadge.json)](https://pdm-project.org)
 
 A utility to fetch and download python packages