Documentation for `ordered_json` should show proper use of the `parse()` function.

https://json.nlohmann.me/features/object_order/ shows inserting objects in a particular order, but doesn't show how to properly use the `parse()` function to get the same result for existing JSON documents.

In particular, it needs to be this:

    auto j = nlohmann::ordered_json::parse(text);

and not this:

    nlohmann::ordered_json j = json::parse(text);
