fix: remove duplicated border-2 class from copy-box

[mattycraig]

boxClasses() already implements a border-2 class by default.

Some background as to why this change matters:

I'm using @nuxtjs/html-validator (https://html-validator.nuxtjs.org/) in a project, and the package is also validating the ./_tailwind colors page, thus giving me a duplicated border-2 class error message on the copy-box div.

As a side question: Is there a way to possibly turn off the generation of the colors via this module?

[codecov]

Codecov Report

Merging #221 (19703a6) into master (15e1c43) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #221   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines           43        43           
  Branches         9         9           
=========================================
  Hits            43        43           

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[atinux]

Thanks @mattycraig

Regarding the way to disable it, could you open an issue about this and the reason why please? :)