03_dispatcher: fix clippy::unnecessary_mut_passed

[softmoth]

Fixes:

warning: the method `dispatch` doesn't need a mutable reference
  --> src/main.rs:70:25
   |
70 |     dispatcher.dispatch(&mut world);
   |                         ^^^^^^^^^^

Checklist

• I've added tests for all code changes and additions (where applicable)
• I've added a demonstration of the new feature to one or more examples
• I've updated the book to reflect my changes
• Usage of new public items is shown in the API docs

API changes