Rm reference to polymorphic data types

VL2/content/VL2_what_you_should_know.tex

@@ -1,5 +1,5 @@
 \begin{itemize}
-\item how to write polymorphic data types and functions
+\item how to write polymorphic functions
 \item how you inline functions
 \item what currying is and why we need it in haskell
 \item how you compose functions