linux/fb.h: Spelling s/palette/palette/

Fix a misspelling of "palette" in a comment.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Helge Deller <deller@gmx.de>