Small improvement to the "Create an API endpoint" dialog

The API is great, and easy to get with this prompt.

Just a tiny fix:
instead of:

curl -H "X-Dune-API-Key:<api_key>" ...

write it as:

curl -H "X-Dune-API-Key:$DUNE_KEY ...

This text is as-understandable, but now it can be directly pasted into the command-line, provided I add the env.var to my shell. Otherwise, I must manually edit it each time.