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

Documentation/filesystems/vfs.rst

@@ -112,7 +112,7 @@ members are defined:
 
 .. code-block:: c
 
-	struct file_system_operations {
+	struct file_system_type {
 		const char *name;
 		int fs_flags;
 		struct dentry *(*mount) (struct file_system_type *, int,