fix: fix to issue apple#132 (implementation of child(through)

`func child(through path: TypedChildIndexPath) -> Markup?` uses the root document node's child count
for comparison against the passed in path for valid reference to a child. Changed it to use the
current iterated element.

fix apple#132

Sources/Markdown/Base/Markup.swift

@@ -265,7 +265,7 @@ extension Markup {
     public func child(through path: TypedChildIndexPath) -> Markup? {
         var element: Markup = self
         for pathElement in path {
-            guard pathElement.index <= raw.markup.childCount else {
+            guard pathElement.index <= element.childCount else {
                 return nil
             }