doc: use parenthesis instead of em dash

Using an em dash has all sorts of typographical pitfalls. For example, in this case you've put spaces around the em dash, but Microsoft and many other style guides say that an em dash should not have spaces around it. As another example, this is two dashes put together rather than an em dash character (—). This is how it was done on mechanical typewriters that didn't have an em dash character, but now we have em dash characters! 🎉

In contrast, the conventions with parentheses are broadly understood and agreed upon.

Originally posted by @trott in https://github.com/nodejs/node/pull/42198#discussion_r818839342