var to const when posssible

exercises/010_if2.zig

@@ -1,16 +1,16 @@
 //
 // If statements are also valid expressions:
 //
-//     var foo: u8 = if (a) 2 else 3;
+//     const foo: u8 = if (a) 2 else 3;
 //
 const std = @import("std");
 
 pub fn main() void {
-    var discount = true;
+    const discount = true;
 
     // Please use an if...else expression to set "price".
     // If discount is true, the price should be $17, otherwise $20:
-    var price: u8 = if ???;
+    const price: u8 = if ???;
 
     std.debug.print("With the discount, the price is ${}.\n", .{price});
 }