Fix midilearn scale example
Fix midilearn scale example

Summary:
After moving the `single` method into the `User` trait, it is now
necessary to use this trait in the example.

Test Plan: Verify that 'cargo test' now works in midilearn.

Reviewers: d3zd3z

Reviewed By: d3zd3z

Differential Revision: https://davidb.phacility.com/D19

d3zd3z/stenome,d3zd3z/stenome