fix deprecation warning in additional_attributes example

[rsnively]

The example code in additional_attributes brings up a deprecation warning related to issue #132 .

This change updates the example code to use strum_macros::Display instead of strum_macros::ToString (as well as removing some unneeded use statements).

Current example code with warning:

Fixed:

[rsnively]

I think this was supposed to make it in with commit #180 but accidentally got overwritten in commit #182

[Peternator7]

thanks!