ports(7): Update instructions for package target
Packages default to ending up in a different location compared to the
documentation, so catch up to the implementation by referring to the
location where packages can usually be found if no environment variables
have been set.
While here, also update the mention of the file extension to match the
txz format that packages use.
PR: 253179, 224370
Reported by: rwatson, jeromer at fastmail dotnet