change the recipe for jenkem to sewage + urine

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You can make saltpetre though: you just must first mix potash before adding piss or compost, or you'll otherwise end up making jenkem. I think it is a good and harmless little reminder of how chem mixing order can matter. I wouldn't mind if pee+sewage and pee+compost was for jenkem, but I'd say keep pee+compost->jenkem.