tests(tests_shutil): fix comment with check_args_to_onerror

- `os.unlink` is used since 2100b42.
- `os.rmdir` is used since 67be92b.

(Note that the special case for Linux/ZFS is not covered [1], and might
actually not work anymore since the changes around handling/setting
`self.errorState`.)

1: https://codecov.io/gh/python/cpython/src/492d513ccb/Lib/test/test_shutil.py#L354...355

Ref: https://bugs.python.org/issue4489

Author: blueyed

Lib/test/test_shutil.py

@@ -339,8 +339,8 @@ def check_args_to_onerror(self, func, arg, exc):
         # on a directory that is chmod 500, which will fail.
         # This function is run when shutil.rmtree fails.
         # 99.9% of the time it initially fails to remove
-        # a file in the directory, so the first time through
-        # func is os.remove.
+        # the file or directory in the directory, so func
+        # is os.unlink or os.rmdir.
         # However, some Linux machines running ZFS on
         # FUSE experienced a failure earlier in the process
         # at os.listdir.  The first failure may legally