Nature is wonderful but it ain't perfect. Think of things like that as slight imperfections and nothing to really worry about.
Deficiencies and other plant problems affect the whole plant or many leaves on the plants. Like low N or Mg will cause yellowing of all the lower and oldest fan leaves and low iron will affect the newest growth all over the plant.
One or two bad leaves does not a problem make. Still healthy looking so best to leave it on to help make food for the rest of the plant. Eventually it will probably be pulled off anyway so not to worry.
I found a great spot to download
FREE POT BOOKS. I downloaded a grow bible first and got lots more. Books look great and complete like the real ones I have here. No web site but just a page of links. Just right click on what you want and then "Save Link As" to download so they don't open first as some are 50+ megs. They got lots. Enjoy.