How to change name of result of rest export?
The HTTP header Content-Disposition will suggest the filename the browser will save as. What's needed here is to make the REST export to add the HTTP header. 

It turns out there's an existing issue for the CSV Serialization module. The current patch in the issue will take the last part of the URL after the slash and appends .csv to it (e.g. 2.csv in your example). You could modify the patch to use whatever filename you want to use for your CSV export.