Update ChimpLineByLine.rst

super minor: use `==` for int comparison in the example (I see this was already fixed in the source)

docs/reST/tut/ChimpLineByLine.rst

@@ -107,7 +107,7 @@ look at each function individually in this section. ::
           raise SystemExit(message)
       image = image.convert()
       if colorkey is not None:
-          if colorkey is -1:
+          if colorkey == -1:
               colorkey = image.get_at((0, 0))
           image.set_colorkey(colorkey, RLEACCEL)
       return image, image.get_rect()