Add slow marker in run/skip option example.

The example implementation of a `--runslow` option results in
a `PytestUnknownMarkWarning`. Include registering the custom
mark in the example, based on the documentation in markers.rst.

doc/en/example/simple.rst

@@ -157,6 +157,10 @@ line option to control skipping of ``pytest.mark.slow`` marked tests:
         )
 
 
+    def pytest_configure(config):
+        config.addinivalue_line("markers", "slow: mark test as slow to run")
+
+
     def pytest_collection_modifyitems(config, items):
         if config.getoption("--runslow"):
             # --runslow given in cli: do not skip slow tests