[drake_ament_cmake_installed] Broken link in README

[BetsyMcPhail]

In the drake_ament_cmake_installed README, the ament_cmake link is broken.

[ahcorde]

The right link is https://docs.ros.org/en/rolling/How-To-Guides/Ament-CMake-Documentation.html

you might want to change the ROS distro branch

[jwnimmer-tri]

Great, thanks!

Since the example is hard-coded to Humble, let's use this link:

• https://docs.ros.org/en/humble/How-To-Guides/Ament-CMake-Documentation.html

When we upgrade from Humble to something newer, we can change the docs and setup scripts at the same time.

+@BetsyMcPhail could you or @nicolecheetham please open a PR for this?

[jwnimmer-tri]

Fixed in #337.