boldface instructions

@ghlai9665 in #348 suggested that:

> By the way, I think it might be a good idea to incorporate format (e.g. boldface) in these instructions to help the model know which words to pay particular "attention" to (just like for humans).
>
> For example: "In this task, we ask you to rewrite the sentence in simple English without changing its general meaning. Essentially, you want to make the sentence [bold] easier to read [/bold] by using simpler words, utilizing more straightforward sentence structures, and omitting non-essential information etc."