update description in Documentation/filesystems/vfs.txt

Update the description of struct file_system_type and get_sb() in
Documentation/filesystems/vfs.txt to match the current code.

Signed-off-by: Borislav Petkov <bbpetkov@yahoo.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>