Consistent instructions and examples

I started off with "hints" that required the poor student to piece
together the information from incomplete bits. A complete example is
like a picture that is worth 1000 words and far clearer.

12_while2.zig

@@ -23,11 +23,13 @@ const std = @import("std");
 pub fn main() void {
     var n: u32 = 2;
 
+    // Please set the continue expression so that we get the desired
+    // results in the print statement below.
     while (n < 1000) : ??? {
         // Print the current number
         std.debug.print("{} ", .{n});
     }
 
-    // Make this print n=1024
+    // As in the last exercise, we want this to result in "n=1024"
     std.debug.print("n={}\n", .{n});
 }