CURLOPT_HEADERFUNCTION.3: fix typo

Refer to _HEADERDATA not _WRITEDATA.

Reported-by: Michał Piechowski

docs/libcurl/opts/CURLOPT_HEADERFUNCTION.3

@@ -85,7 +85,7 @@ static size_t header_callback(char *buffer, size_t size,
                               size_t nitems, void *userdata)
 {
   /* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
-  /* 'userdata' is set with CURLOPT_WRITEDATA */
+  /* 'userdata' is set with CURLOPT_HEADERDATA */
   return nitems * size;
 }