net::ssl::stream is canonical in examples

We no longer need to use beast::ssl_stream to improve write performance, as
Asio now linearizes scatter/gather I/O in ssl::stream:
https://github.com/chriskohlhoff/asio/commit/17637a48ccbfa2f63941d8393a7c8316a8df4a79

example/http/client/async-ssl/http_client_async_ssl.cpp

@@ -17,14 +17,12 @@
 
 #include <boost/beast/core.hpp>
 #include <boost/beast/http.hpp>
-#include <boost/beast/ssl.hpp>
 #include <boost/beast/version.hpp>
+#include <boost/asio/ssl.hpp>
 #include <boost/asio/strand.hpp>
 #include <cstdlib>
-#include <functional>
 #include <iostream>
 #include <memory>
-#include <string>
 
 namespace beast = boost::beast;         // from <boost/beast.hpp>
 namespace http = beast::http;           // from <boost/beast/http.hpp>
@@ -45,7 +43,7 @@ fail(beast::error_code ec, char const* what)
 class session : public std::enable_shared_from_this<session>
 {
     tcp::resolver resolver_;
-    beast::ssl_stream<beast::tcp_stream> stream_;
+    ssl::stream<beast::tcp_stream> stream_;
     beast::flat_buffer buffer_; // (Must persist between reads)
     http::request<http::empty_body> req_;
     http::response<http::string_body> res_;