I'd introduce _the correct Scala way_ first, without the execution context. I understand that you want to cater to a Java audience, but our standard approach is to show the Scala way first and then later show how you can interoperate with Java. I don't think it makes sense to make this distinction at all at this point in the document.

I also think the order of this is wrong – I think it's important to show what a future basically is (ie, the basic idea behind it) and then to introduce what the words "execution context" means. A first-time reader wouldn't know that the execution context that you're showing how to pass is even a thread pool. Parallelism or asynchrony have not been mentioned at all so far in your suggested edits.