USWDS-Site: Replace outdated copySetup references with copyTheme

[amyleadem]

Summary

Replaced copySetup references with copyTheme.

Related issue

Closes #2070

Preview link

Preview link: Phase 2 - Compile page

Problem statement

The Phase 2 Compile guide lists the copySetup gulp task, but it is now called copyTheme in uswds-compile.

Link to copyTheme in uswds-compile's gulpfile.js: https://github.com/uswds/uswds-compile/blob/develop/gulpfile.js#L261

Solution

Replaced all copySetup references with copyTheme

Testing and review

1. Confirm that copyTheme is the correct name of the gulp task
2. Confirm that all copySetup references have been updated

[mahoneycm]

Thanks for taking care of this!

• Confirmed copySetup task in gulp has been updated to copyTheme
• Confirmed the update on site reflects this change in the gulp tasks name
• Confirmed there are no other references to copySetup on site