Update CONTRIBUTING.md now we use dependency groups (#517)

Author: AlexWaygood

CONTRIBUTING.md

@@ -25,7 +25,7 @@ however, we advise setting up a virtual environment first:
 
     $ python3 -m venv env
     $ source env/bin/activate
-    $ pip install -e .[dev]
+    $ pip install -e --group=dev
 
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