Update README (backport #80)

[mergify]

Branch links and build instructions were not correct

Fixes #79

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This is an automatic backport of pull request #80 done by Mergify.

[mergify]

Cherry-pick of 74169e2 has failed:

On branch mergify/bp/humble/pr-80
Your branch is ahead of 'origin/humble' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 74169e2.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   Universal_Robots_ROS2_Gazebo_Simulation.iron.repos

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally