docs: filesystems: vfs: Correct the struct name

The struct name should be file_system_type instead of
file_system_operations.

Signed-off-by: Liao Pingfang <winndows@163.com>
Link: https://lore.kernel.org/r/1610265599-5101-1-git-send-email-winndows@163.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>