docs: fix link to up CLI

[mikaello]

Description of your changes

http://github.com/upbound/up seems to be a private repo, so it is better to point users to open sources to get the CLI tool.

Fixes #30

I have:

• Read and followed Upbound's contribution process.

• Run make reviewable to ensure this PR is ready for review.
  No this didn't work:

  $ make reviewable
  make: *** No rule to make target 'reviewable'.  Stop.
  

  This may be in one of the submodules, so I tried make submodules, but this failed as well:

  ...
  git@github.com: Permission denied (publickey).
  fatal: Could not read from remote repository.
  
  Please make sure you have the correct access rights
  and the repository exists.
  

• Added backport release-x.y labels to auto-backport this PR, as appropriate.
  Not appropriate in this case, I think.

How has this code been tested

URLs are being checked to ensure they are accessible to everyone.

[Upbound-CLA]

All committers have signed the CLA.