:memo: fix example

docs/examples/to_bon8.cpp

diff --git a/docs/examples/to_bon8.cpp b/docs/examples/to_bon8.cpp
index 2b4e311..2f26342 100644
--- a/docs/examples/to_bon8.cpp
+++ b/docs/examples/to_bon8.cpp
@@ -3,6 +3,7 @@
 #include <nlohmann/json.hpp>
 
 using json = nlohmann::json;
+using namespace nlohmann::literals;
 
 int main()
 {