doc: fix pytest_load_initial_conftests example referring to conftest.py

The 'Dynamically adding command line options' example defines
pytest_load_initial_conftests under a '# installable external plugin' comment
but the following prose called it 'the above conftest.py'. This contradicts the
hook reference, which states the hook is not called for conftest files. Refer to
it as a plugin instead.

Closes #4482

Author: PranavAchar01

changelog/4482.doc.rst

@@ -0,0 +1,5 @@
+Corrected the "Dynamically adding command line options" example so it no longer
+refers to the ``pytest_load_initial_conftests`` hook as living in a
+``conftest.py``. As noted in the hook reference, ``pytest_load_initial_conftests``
+is not called for ``conftest.py`` files and must be provided by an installable
+plugin.

doc/en/example/simple.rst

@@ -229,7 +229,7 @@ the command line arguments before they get processed:
 If you have the :pypi:`xdist plugin <pytest-xdist>` installed
 you will now always perform test runs using a number
 of subprocesses close to your CPU. Running in an empty
-directory with the above conftest.py:
+directory with the above plugin:
 
 .. code-block:: pytest