README: fix nix-channel --update command

Following the current instructions yields these errors for some reason:

```
$ sudo nixos-rebuild switch
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I), at (string)13
building Nix...
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos/modules/installer/tools/nix-fallback-paths.nix' was not found in the Nix search path (add it using $NIX_PATH or -I)
/tmp/nixos-rebuild.vs4JVw/nix
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
building the system configuration...
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
error: file 'nixpkgs/nixos' was not found in the Nix search path (add it using $NIX_PATH or -I)
```

README.md

@@ -6,7 +6,7 @@ Add and update `nixos-hardware` channel:
 
 ```
 $ sudo nix-channel --add https://github.com/NixOS/nixos-hardware/archive/master.tar.gz nixos-hardware
-$ sudo nix-channel --update nixos-hardware
+$ sudo nix-channel --update
 ```
 
 Then import an appropriate profile path from the table below. For example, to