Add missing opening braces in while statements

exercises/12_while2.zig

@@ -4,14 +4,14 @@
 // end of the loop or when an explicit 'continue' is invoked (we'll
 // try those out next):
 //
-//     while (condition) : (continue expression)
+//     while (condition) : (continue expression) {
 //         ...
 //     }
 //
 // Example:
 //
 //     var foo = 2;
-//     while (foo<10) : (foo+=2)
+//     while (foo<10) : (foo+=2) {
 //         // Do something with even numbers less than 10...
 //     }
 //