docs: filesystems: vfs: Use SPDX identifier

Currently the licence is indicated via a custom string.  We have SPDX
license identifiers now for this task.

Use SPDX license identifier matching current license string.
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/vfs.txt

+.. SPDX-License-Identifier: GPL-2.0
+
 =========================================
 Overview of the Linux Virtual File System
 =========================================
@@ -7,8 +9,6 @@ Overview of the Linux Virtual File System
   Copyright (C) 1999 Richard Gooch
   Copyright (C) 2005 Pekka Enberg
 
-  This file is released under the GPLv2.
-
 
 Introduction
 ============