Change picture and prep time of lasagna. (#495)

I just made it and took a nice picture I'd like to add.

Also, I changed
the preperation time to only the time this recipe takes. I think that's
better, because we don't know whether the sauces are store-bought or
not.

src/lasagna.md

@@ -4,7 +4,7 @@
 
 A nice recipe for amazing fully home-made lasagna.
 
-- ⏲️ Prep time: 4 hours 30 minutes
+- ⏲️ Prep time: 1 hour
 - 🍳 Cook time: 45 min
 - 🍽️ Servings: 6