Update URL: python/black → psf/black (#477)

Author: jdufresne

README.rst

@@ -12,7 +12,7 @@ The CPython Developer's Guide
    :target: https://python.zulipchat.com
 
 .. |Codestyle| image:: https://img.shields.io/badge/code%20style-black-000000.svg
-   :target: https://github.com/ambv/black
+   :target: https://github.com/psf/black
    :alt: Code style is black