Fix a CI_Input::set_cookie() bug

Found on StackOverflow: https://stackoverflow.com/questions/41925028/codeigniter-config-overrides-set-cookie-parameters
diff --git a/user_guide_src/source/helpers/cookie_helper.rst b/user_guide_src/source/helpers/cookie_helper.rst
index c9d2f41..2ad51e7 100644
--- a/user_guide_src/source/helpers/cookie_helper.rst
+++ b/user_guide_src/source/helpers/cookie_helper.rst
@@ -25,7 +25,7 @@
 The following functions are available:
 
 
-.. php:function:: set_cookie($name[, $value = ''[, $expire = ''[, $domain = ''[, $path = '/'[, $prefix = ''[, $secure = FALSE[, $httponly = FALSE]]]]]]])
+.. php:function:: set_cookie($name[, $value = ''[, $expire = ''[, $domain = ''[, $path = '/'[, $prefix = ''[, $secure = NULL[, $httponly = NULL]]]]]]])
 
 	:param	mixed	$name: Cookie name *or* associative array of all of the parameters available to this function
 	:param	string	$value: Cookie value