Fix docs references to flux.yaml

In the file docs/guides/prerequisites/install_flux.md the command kubectl apply -f https://repo1.dso.mil/platform-one/big-bang/bigbang/-/raw/master/scripts/deploy/flux.yaml results in a 404. Seems like flux.yaml may have been moved in commit https://repo1.dso.mil/platform-one/big-bang/bigbang/-/commit/5f0ee7e5f1fb81be07c646c78d3a5f9d1b5ae6eb