update playground.rs to align with docs [chore]

[sanderjson]

Fix bug

error[E0422]: cannot find struct, variant or union type `ActiveModel` in this scope
  --> examples/playground.rs:10:47
   |
10 |     let active_model: articles::ActiveModel = ActiveModel {
   |                                               ^^^^^^^^^^^ not found in this scope
   |
help: consider importing one of these structs
   |
1  + use crate::articles::ActiveModel;
   |
1  + use loco_rs::tests_cfg::db::test_db::ActiveModel;
   |
1  + use myapp_docs_demo::models::articles::ActiveModel;
   |
1  + use myapp_docs_demo::models::users::ActiveModel;
   |

For more information about this error, try `rustc --explain E0422`.
error: could not compile `myapp` (example "playground") due to 1 previous error

[kaplanelad]

Thanks, @sanderjson, for your contribution!

Please ensure that only the changes under loco-new/base_template/examples/playground.rs.t are kept.

The demo will be removed as part of #1156. Additionally, the startes/* directory contains our old Loco templates, which are deprecated.