Alternating with one knit row and one purl row is called stocking stitch or stockinette, and it always curls. You can't really fix it in work that you've already completed, although blocking can sometimes reduce the curl. But preventing it in the future is easy - you just need to add a border of garter st or seed st around the edges. Most people find 3 to 5 sts is sufficient, your mileage may vary.
Just because I'm easily distracted doesn't mean I