updated doc to delete deprecated hook

doc/en/example/simple.rst

@@ -102,7 +102,7 @@ the command line arguments before they get processed:
 
     # content of conftest.py
     import sys
-    def pytest_cmdline_preparse(args):
+    def pytest_load_initial_conftests(args):
         if 'xdist' in sys.modules: # pytest-xdist plugin
             import multiprocessing
             num = max(multiprocessing.cpu_count() / 2, 1)