Fix documentation link in the README. (#626)

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

README.md

@@ -1,7 +1,7 @@
 # rosidl
 
 ```rosidl``` is one of the ros_core packages.
-See [documentation](http://docs.ros2.org/latest/developer_overview.html#the-rosidl-repository) for details of this package.
+See [documentation](https://docs.ros.org/en/rolling/Concepts/About-Internal-Interfaces.html#the-rosidl-repository) for details of this package.
 
 ## Packages