Fix #11438: fix typo in match type docs snippet

Author: OlivierBlanvillain

docs/docs/reference/new-types/match-types.md

@@ -67,7 +67,7 @@ use of the match type as the return type):
 def leafElem[X](x: X): LeafElem[X] = x match
    case x: String      => x.charAt(0)
    case x: Array[t]    => leafElem(x(9))
-   case x: Iterable[t] => leafElem(x.next())
+   case x: Iterable[t] => leafElem(x.head)
    case x: AnyVal      => x
 ```