Tweak language and formatting in protein-translation/description.md

[codingthat]

• Remove extraneous => for consistency
• Remove redundant "after" (given "subsequent")
• Wikipedia just calls them "stop codons," not "STOP codons," but I italicized instead as a way to introduce a new term

Discussion on the forum is at: https://forum.exercism.org/t/tweak-language-and-formatting-in-protein-translation-description-md/17005/16

[codingthat]

Interesting, the link checker 429'd. 🤔