Update header callback to use const

Co-authored-by: Fabian Sauter <sauter.fabian@mailbox.org>

Author: Jacoblightning

advanced-usage.md

@@ -532,7 +532,7 @@ This includes empty lines and the `HTTP` status line.  `\r\n` endings are preser
 The callback signature looks like this.
 
 ```c++
-  bool headerCallback(std::string_view & data, intptr_t userdata);
+  bool headerCallback(const std::string_view & data, intptr_t userdata);
 ```
 
 Provide the callback with the HeaderCallback options object.  Only one header callback may be set.