nixos/doc: clarify command template parameter

squash-reworded from #284844

Changed files
+3 -1
nixos
doc
+3 -1
nixos/doc/manual/installation/installing-from-other-distro.section.md
···
will be safer to use the `nixos-*` channels instead:
```ShellSession
-
$ nix-channel --add https://nixos.org/channels/nixos-version nixpkgs
+
$ nix-channel --add https://nixos.org/channels/nixos-<version> nixpkgs
```
+
+
Where `<version>` corresponds to the latest version available on [channels.nixos.org](https://channels.nixos.org/).
You may want to throw in a `nix-channel --update` for good measure.