## Description

This updates the copy icon for copying sample code to use `content_copy` instead of `assignment`.
Before:
<img width="369" alt="223876418-2470c4c5-abfc-4511-9762-1fd0b6e05903" src="https://user-images.githubusercontent.com/8867023/228396142-450125ee-d8cf-4ede-8545-5920eb0da99d.png">

After:
<img width="408" alt="image" src="https://user-images.githubusercontent.com/8867023/228395959-8531704a-77aa-43af-9dcf-7456bb2ed090.png">

## Related Issues
 - Fixes https://github.com/flutter/flutter/issues/122246

## Tests
 - No tests needed.