functional/stateful-types.md: small wording tweak

Author: jhwgh1968

functional/stateful-types.md

@@ -10,9 +10,10 @@ In C++ and Java, for example, generic types a meta-programming construct for the
 compiler. A `Vec<int>` and `Vec<char>` in C++ are just two different copies of
 the same boilerplate code for a `Vec` type, with two different types filled in.
 
-In Rust, the generic type parameter creates what is known as a "type class", and
-each value used by an end user *actually changes the type of each instatiation*.
-In other words, `Vec<usize>` and `Vec<char>` *are two different types*.
+In Rust, the generic type parameter creates what is known as a "type class
+constraint", and each value used by an end user *actually changes the type of
+each instatiation*.  In other words, `Vec<usize>` and `Vec<char>` *are two
+different types*.
 
 This is why `impl` blocks must specify generic parameters: different ones can
 have different `impl` blocks on them.