Fixed minor typos in the "Understanding Optics" section

In the section "Prism" section some references to "product type" were left behind, when "sum type" is the relevant one.

docs/understanding-optics.md

@@ -53,7 +53,7 @@ We use the data type `OpticFailure` to model the different ways that getting or
 trait OpticFailure extends Throwable
 ```
 
-The `SetError` type of a prism will be `Nothing` because given one of the cases of a product type we can always return a new value of the product type since each case of the product type is an instance of the product type.
+The `SetError` type of a prism will be `Nothing` because given one of the cases of a sum type we can always return a new value of the sum type since each case of the sum type is an instance of the sum type.
 
 A prism also differs from a lens in that we do not need any original structure to set. A product type consists of nothing but its cases so if we have a new value of the case we want to set we can just use that value and don't need the original structure.