Benjamin, the leaves are not showing a clear direct pattern yet, as fortunately the deficiency is still slight. It is good you noticed the concern quickly. The leaf's greenish center with yellow sides and top is typical of the start of a Magnesium deficiency. This can be corrected by dissolving one teaspoon of Epsom Salts (Magnesium Sulfate) in a gallon of WARM water, as Epsom salts does not dissolve well in cold water. There is also a slighter chance it could be a deficiency in nitrogen, but I tend to think it is indeed magnesium.