[Docs] Remove dead link to CentOS 8 Dockerfile
[Docs] Remove dead link to CentOS 8 Dockerfile

CentOS 8 is no longer supported after https://github.com/apple/swift-docker/pull/273
benlangmuir/swift,JGiola/swift,ahoppen/swift,ahoppen/swift,glessard/swift,atrick/swift,glessard/swift,glessard/swift,apple/swift,apple/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,ahoppen/swift,atrick/swift,ahoppen/swift,roambotics/swift,gregomni/swift,gregomni/swift,gregomni/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,atrick/swift,gregomni/swift,apple/swift,glessard/swift,atrick/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,apple/swift,JGiola/swift,gregomni/swift,ahoppen/swift,roambotics/swift,atrick/swift,ahoppen/swift,roambotics/swift,benlangmuir/swift,apple/swift,atrick/swift,JGiola/swift,glessard/swift,gregomni/swift,roambotics/swift