Update docs README.md install command. (#563)

Author: jonathan343

codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/SetupGenerator.java

@@ -471,7 +471,7 @@ private static void writeDocsReadme(
                     following:
 
                     ```
-                    $$ uv pip install ".[docs]"
+                    $$ uv pip install --group docs .
                     $$ cd docs
                     $$ make html
                     ```