docs: fs: fix broken links to vfs.txt with was renamed to vfs.rst

A recent documentation conversion renamed this file but forgot
to update the links.

Fixes: af96c1e3 ("docs: filesystems: vfs: Convert vfs.txt to RST")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>