Correct regression-palette-8.png

This corrects contrib/testpngs/badpal/regression-palette-8.png.  Despite
the comment in commit da109d3e the file checked in had 255 PLTE entries
so failed to perform the regression test.  This version of the PNG has
254 entries.

Signed-off-by: John Bowler <jbowler@acm.org>
1 file changed