Merge pull request #185069 from fourplusone/patch-1

Correct instructions to obtain a hash for git repos

doc/contributing/coding-conventions.chapter.md

···
   }
   ```
 
-  Find the value to put as `sha256` by running `nix run -f '<nixpkgs>' nix-prefetch-github -c nix-prefetch-github --rev 1f795f9f44607cc5bec70d1300150bfefcef2aae NixOS nix` or `nix-prefetch-url --unpack https://github.com/NixOS/nix/archive/1f795f9f44607cc5bec70d1300150bfefcef2aae.tar.gz`.
+Find the value to put as `sha256` by running `nix-shell -p nix-prefetch-github --run "nix-prefetch-github --rev 1f795f9f44607cc5bec70d1300150bfefcef2aae NixOS nix"`. 
 
 ## Obtaining source hash {#sec-source-hashes}