README.md: Duplicate example for starting existing nixosConfigurations

[preisi]

The readme mentions the existence of two different variants for starting VMs for existing nixosConfigurations. However, the listed commands in

nixos-shell/README.md

Line 276 in 671f301

nix run .#nixosConfigurations.<yourmachine>.config.system.build.nixos-shell

and

nixos-shell/README.md

Line 282 in 671f301

nix run .#nixosConfigurations.<yourmachine>.config.system.build.nixos-shell

are identical.