Documentation/filesystems/vfat.txt: fix a remark that implies UCS2

All non-historic operating systems support the full range of Unicode here,
thus you can make filenames for example in Gothic (𐌼𐌴𐍉𐍅), the other Gothic
(𝓂ℯℴ𝓌) or the third Gothic (𝗆𝖾𝗈𝗐), or declare something as 💩

.

Characters above U+FFFF are encoded on four bytes.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>