Userland: Fix remaining smart pointer const-correctness issues

Author: https://github.com/awesomekling
Commit: 33e87d1627
Pull-request: https://github.com/SerenityOS/serenity/pull/17557
Reviewed-by: https://github.com/linusg

Userland/Applications/Help/ManualModel.cpp

@@ -149,7 +149,7 @@ GUI::ModelIndex ManualModel::parent_index(const GUI::ModelIndex& index) const
         return {};
     auto children = maybe_children.release_value();
     for (size_t row = 0; row < children.size(); row++) {
-        Manual::Node* child_at_row = children[row];
+        Manual::Node const* child_at_row = children[row];
         if (child_at_row == parent)
             return create_index(row, 0, parent);
     }