Don't use deprecated API in docs

include/fmt/format.h

@@ -607,8 +607,8 @@ enum { inline_buffer_size = 500 };
 
   **Example**::
 
-     fmt::memory_buffer out;
-     format_to(out, "The answer is {}.", 42);
+     auto out = fmt::memory_buffer();
+     format_to(std::back_inserter(out), "The answer is {}.", 42);
 
   This will append the following output to the ``out`` object: