fixed wrong openscad function syntax in examples/scad_to_include.scad

Author: jeff-dh

solid/examples/scad_to_include.scad

@@ -17,9 +17,7 @@ function scad_points() = [[0,0], [1,0], [0,1]];
 
 // In Python, calling this function without an argument would be an error.
 // Leave this here to confirm that this works in OpenSCAD.
-function optional_nondefault_arg(arg1){
-    s = arg1 ? arg1 : 1;
-    cube([s,s,s]);
-}
+function optional_nondefault_arg(arg1) =
+  let(s = arg1 ? arg1 : 1) cube([s,s,s]);
 
 echo("This text should appear only when called with include(), not use()");