📝 fixed examples for Wandbox

As I learned in https://github.com/melpon/wandbox/issues/209, this
library is already installed at Wandbox, so we need to adjust the
examples to use `#include "json.hpp"` insteas of `#include <json.hpp>`.

doc/examples/rbegin.cpp

@@ -1,4 +1,4 @@
-#include <json.hpp>
+#include "json.hpp"
 
 using json = nlohmann::json;