var to const when posssible

exercises/016_for2.zig

@@ -29,7 +29,7 @@ pub fn main() void {
         // Note that we convert the usize i to a u32 with
         // @intCast(), a builtin function just like @import().
         // We'll learn about these properly in a later exercise.
-        var place_value = std.math.pow(u32, 2, @intCast(u32, i));
+        const place_value = std.math.pow(u32, 2, @intCast(u32, i));
         value += place_value * bit;
     }