Is there as way to notify a moderator after you edit an off-topic question to make it on-topic?
I edited this question's title to make it on-topic, but I'm not sure if I need to do something more for a moderator to review my edit.
When a closed question is edited by the asker (as opposed to someone else just fixing it up), it is automatically entered into the 'reopen review queue'. No flagging is necessary.
If it is edited by someone else, it will need to be either flagged (a mod will review the flag), a reopen vote cast by yourself (for those who have 3k rep), or a reopen vote cast by someone else (consider going into chat to ask if anyone can start the ball rolling).
The details of these mechanics are in the MSO answer to Can we have the ability to flag for reopening?
Adding items to the queue in response to 3rd-party edits. Currently, this is done when the author edits, but not if someone else does the same. It would be possible to change that - although there are some pitfalls we'd have to avoid.
In order of preference:
Your edit should start the re-open process automatically, as MichaelT explained. Also, every significant edit bumps the question to the top of the site's front page, so a new round of attention from people casually browsing the site is almost guaranteed.
All five moderators and quite a few other regulars are active in the Whiteboard, the site's main chat room. Even if there's no one around at the time, you can just drop a link to the question and eventually someone will ping you back with their thoughts on your edit. Or, if they happen to agree with the edit and there isn't much to discuss, they'll just vote to re-open it.
If the question fails to get enough re-open votes through the re-open queue and you hadn't had any luck in chat, feel free to start a Meta discussion asking how the question in question can be further improved and re-opened.
What you shouldn't do is flag for moderation attention. The site currently has 366 users who can vote to re-open (3000+ reputation), and most of them are fairly active; there's little need to call one of the five diamonds.