Update nix-tools-pin.md (#1628)

Author: hamishmack

docs/dev/nix-tools-pin.md

@@ -1,16 +1,13 @@
 ## How to update [`nix-tools`](https://github.com/input-output-hk/nix-tools)
 
 1. Use niv to update the sources.json:
-
    ```
-   niv update nix-tools
+   nix flake lock --update-input nix-tools
    ```
 
-
 2. If `nix-tools.cabal` or `plan-to-nix` have changed, check the
    materialized files for each of the compiler nix name in
    `ls -d materialized/ghc*/nix-tools` with:
-
    ```
    nix-build scripts/check-compiler-materialization --argstr compiler-nix-name ghc884
    ```