Update os.fwalk example

Author: totosuki

Doc/library/os.rst

@@ -3727,16 +3727,16 @@ features:
 
    This example displays the number of bytes taken by non-directory files in each
    directory under the starting directory, except that it doesn't look under any
-   CVS subdirectory::
+   ``__pycache__`` subdirectory::
 
       import os
-      for root, dirs, files, rootfd in os.fwalk('python/Lib/email'):
+      for root, dirs, files, rootfd in os.fwalk('python/Lib'):
           print(root, "consumes", end="")
           print(sum([os.stat(name, dir_fd=rootfd).st_size for name in files]),
                 end="")
           print("bytes in", len(files), "non-directory files")
-          if 'CVS' in dirs:
-              dirs.remove('CVS')  # don't visit CVS directories
+          if '__pycache__' in dirs:
+              dirs.remove('__pycache__')  # don't visit __pycache__ directories
 
    In the next example, walking the tree bottom-up is essential:
    :func:`rmdir` doesn't allow deleting a directory before the directory is