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>