As shown, it is not too dangerous, carefully studying the pictures and make a decision, do not rush with fungicides. I tend to believe Sunburn or Bacterial Brown Spot.
Watch it closely for any changes. If the size of the spot increases, it is bacterial rot and you should cut off the leaf with a flame-sterilized/cooled blade (or bleach sterilized and rinsed blade), cutting into healthy green tissue (about 1 inch outside the spot if possible). Bacterial rot can spread quickly and kill an entire plant, and also put other plants nearby at risk.