Unfortunately no, flags are not revocable, but you are not without recourse if you do regret the decision to flag. Here are three options. Before you do any of them, make sure your flag is not already handled by going to your profile and clicking on the number next to the words "helpful flags"
Comment on the post indicating that it was flagged in error.
Ping a mod in chat asking them to ignore the flag
Send a second flag with a custom message indicating that we should ignore previous. (this has the benefit of preserving your anonymity that the other methods do not)
However, regardless of whether or not you choose to notify us that the flag was in error or that something has changed that makes your flag obsolete, keep in mind that we don't act on flags like mindless automatons. We evaluate each flag and try to take the correct action. It helps to know what is wrong, but most of the time a flag is a general alert that something is. If we can see it was fixed or is no longer an issue (or we don't feel the issue is severe enough to require moderator action) your flag may be dismissed without moderator action (either as helpful or declined depending on a number of things).
The request has certainly come up on MSO but as of yet has not been considered for implementation.