Minor naming fixes for ParameterValue to_string() function (ros2#2609)

More appropriate function argument naming.
Refer to: 
https://github.com/ros2/rclcpp/blob/rolling/rclcpp/src/rclcpp/parameter_value.cpp#L83

Signed-off-by: Kang, Hsin-Yi <f039281310@yahoo.com.tw>

rclcpp/include/rclcpp/parameter_value.hpp

@@ -359,7 +359,7 @@ class ParameterValue
 /// Return the value of a parameter as a string
 RCLCPP_PUBLIC
 std::string
-to_string(const ParameterValue & type);
+to_string(const ParameterValue & value);
 
 }  // namespace rclcpp