Improper string formatting in example

[filmorependrgn@…]

doc/html/boost_asio/example/chat/chat_message.hpp has an improper string formatting that can cause compiler noisiness.

[anonymous]

I can't figure out if this is fixed in the latest source, but the change is

84c84
<     sprintf(header, "%4d", body_length_);
---
>     sprintf(header, "%4zd", body_length_);