Document pytest_add_cli_args_test_selection instead of tests_dir

Author: Otto-AA

README.rst

@@ -90,15 +90,15 @@ In `setup.cfg` in the root of your project you can configure mutmut if you need
 
     [mutmut]
     paths_to_mutate=src/
-    tests_dir=tests/
+    pytest_add_cli_args_test_selection=tests/
 
 If you use `pyproject.toml`, you must specify the paths as array in a `tool.mutmut` section:
 
 .. code-block:: toml
 
     [tool.mutmut]
     paths_to_mutate = [ "src/" ]
-    tests_dir = [ "tests/" ]
+    pytest_add_cli_args_test_selection= [ "tests/" ]
 
 See below for more options for configuring mutmut.