langref: convert to unmanaged ArrayList in example

Author: meowjesty

doc/langref/testing_detect_leak.zig

@@ -1,9 +1,10 @@
 const std = @import("std");
 
 test "detect leak" {
- var list = std.array_list.Managed(u21).init(std.testing.allocator);
- // missing `defer list.deinit();`
- try list.append('☔');
+ const gpa = std.testing.allocator;
+ var list: std.ArrayList(u21) = .empty;
+ // missing `defer list.deinit(gpa);`
+ try list.append(gpa, '☔');
 
  try std.testing.expect(list.items.len == 1);
 }