Update doc: pytest section in setup.cfg (#5894)

Update doc: pytest section in setup.cfg

doc/en/customize.rst

@@ -134,10 +134,13 @@ progress output, you can write it into a configuration file:
 .. code-block:: ini
 
     # content of pytest.ini or tox.ini
-    # setup.cfg files should use [tool:pytest] section instead
     [pytest]
     addopts = -ra -q
 
+    # content of setup.cfg
+    [tool:pytest]
+    addopts = -ra -q
+
 Alternatively, you can set a ``PYTEST_ADDOPTS`` environment variable to add command
 line options while the environment is in use: