Correct modifier application description in prefixing.md

[CodeF53]

Modifiers are appended to the colors #e700ff.bold prepending would be like this bold.#e700ff

[coveralls]

coverage: 100.0%. remained the same
when pulling 679656c on CodeF53:patch-1
into 9f56933 on open-cli-tools:main.

[paescuj]

Thanks!