is31fl3731: Fix nextframe index

It looks like this what original author wanted,
if not then next_frame should be set to 0 unconditionally

Forwarded: #19
Signed-off-by: Philippe Coval <philippe.coval@huawei.com>

library/rgbmatrix5x5/is31fl3731.py

@@ -289,7 +289,7 @@ def show(self):
         """Show the buffer contents on the display."""
         self.setup()
 
-        next_frame = 0 if self._current_frame == 1 else 0
+        next_frame = 0 if self._current_frame == 1 else 1
 
         output = [0 for x in range(144)]