There's two things here:
- If you look at the transport logs (and not the "from") you can see in fact the email is from Stack Overflow
- The email itself matches what others receive
On both fronts I can confirm this email looks like an legitimate email from the Stack Exchange network. That being said, it's always possible for a spammer to take that mail content and send it to you with different links - so you can always decline to click any links until you verify them in any email.
Many emails generated by the network (e.g. newsletters, etc) will be sent from Stack Overflow, so this is not a concern even though it's referencing your Ask Different registration.
As for the email itself, I've checked and can confirm this is a legitimate email that goes out to new users if they achieve a certain amount of reputation within a particular period.
In short, this looks very much like nothing to be concerned about and, in fact, you can pat yourself on the back for being an active member of the community as well as knowing that phishing mails can be sent that look great.
Most importantly, Thank you for your contributions here!