nix-channel documentation: don't suggest deprecated function

Running `nix-instantiate --eval -E '(import <nixpkgs> {}).lib.nixpkgsVersion` emits a warning
```
trace: `lib.nixpkgsVersion` is deprecated, use `lib.version` instead!
```
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Nathan van Doorn 2018-07-16 10:00:42 +01:00 committed by GitHub
parent 1b34b69b45
commit 41f38fbb4b
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@ -112,13 +112,13 @@ $ nix-env -iA nixpkgs.hello</screen>
<para>You can revert channel updates using <option>--rollback</option>:</para>
<screen>
$ nix-instantiate --eval -E '(import &lt;nixpkgs> {}).lib.nixpkgsVersion'
$ nix-instantiate --eval -E '(import &lt;nixpkgs> {}).lib.version'
"14.04.527.0e935f1"
$ nix-channel --rollback
switching from generation 483 to 482
$ nix-instantiate --eval -E '(import &lt;nixpkgs> {}).lib.nixpkgsVersion'
$ nix-instantiate --eval -E '(import &lt;nixpkgs> {}).lib.version'
"14.04.526.dbadfad"
</screen>