Documentation/filesystems/vfat.txt: fix directory entry example

Slots can store up to 13 characters for the file name but one of the
examples has one character too many.

Signed-off-by: Luis Ortega Perez de Villar <luiorpe1@upv.es>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>