How do I correct acidity/tartness in a fermented beer?

You could blend the beer with maltier/sweeter beer in the glass to change your perception. Different brewing salts may help but they some are not easily dissolved into cold and carbonated beer, if in fact that's the status of your beer now.

In general I always find it better to learn what went wrong and try and fix that than fix the beer. Time better spent preparing for the next batch would be better spent.

There are a couple things you can try adding to a glass of the beer. The sodium and chloride in salt will aid in the perception of sweetness, so you could try adding a bit to a glass. Too much, though, will obviously give you a salty flavor. You can also add calcium chloride to the glass to enhance the perception if maltiness and sweetness. Again, start with just a tiny bit and add more if necessary. If what you're experiencing is astringency due to a high pH, though, these might be of limited usefulness. But they're at least worth a try.

You could lower the carbonation level of the keg slightly. Carbonic acid comes across as 'tart', so backing off the C02 could help the beer feel more normal.

Also, you could try adding small amounts of Malto Dextrin powder to the finished beer. Malto Dextrin is normally used to increase body and mouthfeel, but it might have the side effect of cutting some of the tartness. Add it slowly, as I doubt you'd want to get the FG higher than 1.013 or so, otherwise it might start feeling "chewy".