Update installing.rst

[jolsso]

Changed CentOS directory to use nightly rather than latest, since the files are only available in nightly.

[MarkRiddoch]

Hi Jolsso, firstly, thank you for your contribution. Could I please ask your to sign your commit? it is a requirement of contributions to Linux Foundation projects that all contributions are signed, as per point 4 in the contributing.md file

Commit your changes in logical chunks. When you are ready to commit, make sure to write a Good Commit Message™. Use interactive rebase to group your commits into logical units of working before making them public.

Note that every commit you make must be signed. By signing off your work you indicate that you are accepting the Developer Certificate of Origin.

Use your real name (sorry, no pseudonyms or anonymous contributions). If you set your user.name and user.email git configs, you can sign your commit automatically with git commit -s.