Documentation/fb/vesafb.txt: fix typo

Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Cc: Gerd Knorr <kraxel@goldbach.in-berlin.de>
Cc: Nico Schmoigl <schmoigl@rumms.uni-mannheim.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>