Update generics: use xs.prepended(x) instead of x :: xs (#2178)

Author: AesaKamar

_overviews/scala3-book/types-generics.md

@@ -20,7 +20,7 @@ class Stack[A]:
   //                         ^
   //  Here we refer to the type parameter
   //          v
-  def push(x: A): Unit = { elements = x :: elements }
+  def push(x: A): Unit = { elements = elements.prepended(x) }
   def peek: A = elements.head
   def pop(): A =
     val currentTop = peek