Change variant reference from boost to C++17

Updated reference from `boost::variant` to `std::variant` in C++17.

Author: ktgw0316

src/content/1.5/products-and-coproducts.tex

@@ -465,8 +465,8 @@ \section{Coproduct}
 It injects an integer into \code{Contact}.
 
 A tagged union is also called a \newterm{variant}, and there is a very
-general implementation of a variant in the boost library,
-\code{boost::variant}.
+general implementation of a variant in the C++17 standard,
+\code{std::variant}.
 
 In Haskell, you can combine any data types into a tagged union by
 separating data constructors with a vertical bar. The \code{Contact}