docs: Fix versioned archive path
docs: Fix versioned archive path

Fix the version path for the inner directory inside the archive for a
given tree version of the source code fetched from github archives.

Example URL: https://github.com/cilium/cilium/archive/v1.6.tar.gz
Inner directory: cilium-1.6

Prior to this commit, we would generate doc instructions pointing
users to "cd cilium-v1.6" which doesn't exist.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tgraf/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,cilium-team/cilium