Replace Black with Ruff in contributing.rst. (#6903)

docs/contributing.rst

@@ -251,10 +251,10 @@ Coding style
 Please follow these coding conventions when writing code for inclusion in
 Scrapy:
 
-* We use `black <https://black.readthedocs.io/en/stable/>`_ for code formatting.
+* We use `Ruff <https://docs.astral.sh/ruff/>`_ for code formatting.
   There is a hook in the pre-commit config
   that will automatically format your code before every commit. You can also
-  run black manually with ``tox -e pre-commit``.
+  run Ruff manually with ``tox -e pre-commit``.
 
 * Don't put your name in the code you contribute; git provides enough
   metadata to identify author of the code.