fix: nix shell multiple commands example (#8950)

The `-c` flag belongs to `sh` not `nix shell`. As it stands, the command errors with:

```
$ nix shell nixpkgs#gnumake --command sh --command "cd src && make"
sh: --command: invalid option
```

https://github.com/NixOS/nix/pull/8276 was good for readability, but it missed this since that PR used a find/replace script.

src/nix/shell.md

@@ -26,7 +26,7 @@ R""(
 * Run multiple commands in a shell environment:
 
   ```console
-  # nix shell nixpkgs#gnumake --command sh --command "cd src && make"
+  # nix shell nixpkgs#gnumake --command sh -c "cd src && make"
   ```
 
 * Run GNU Hello in a chroot store: