📝 fixing documentation #867

The example in the documentation on how to "force" an array of arrays was wrong since the first release. Fixed the documentation and added checks for the README unit tests.

README.md

@@ -160,7 +160,7 @@ json empty_object_implicit = json({});
 json empty_object_explicit = json::object();
 
 // a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
-json array_not_object = { json::array({"currency", "USD"}), json::array({"value", 42.99}) };
+json array_not_object = json::array({ {"currency", "USD"}, {"value", 42.99} });
 ```