Clarify $() and ${}

[kinow]

Fixes common-workflow-language/common-workflow-language#206

Supersedes common-workflow-language/common-workflow-language#929

Quoting the superseded PR:

Hi,

Having a go at the Barn Raising issue about ES expression and ES function body. At first I wrote a longer version with an example $(new Date()) for expression, and a longer ${var dt = new Date(); dt.setDate(dt.getDate() - 2); return dt} function body that returns a date minus 2 days.

But I think it became too specific and made the section even longer? I also thought that perhaps the comparison could go into a NOTE section, but didn't find any such section nearby, so went with the simpler approach.

Happy to amend as per feedback 👍

Thanks
Bruno

p.s.: this PR also fixes a typo, "EMACScript" where it should be "ECMAScript".