[suggested.md] changelog-seen -> change-id in shell.nix`

`changelog-seen` was deprecated in rust-lang#115898

```
WARNING: The use of `changelog-seen` is deprecated. Please refer to `change-id` option in `config.example.toml` instead.
WARNING: The `change-id` is missing in the `config.toml`. This means that you will not be able to track the major changes made to the bootstrap configurations.
note: to silence this warning, add `change-id = 115898` at the top of `config.toml`
```

Author: eopb

src/doc/rustc-dev-guide/src/building/suggested.md

@@ -277,7 +277,7 @@ let
   # `config.toml.example`) from `1bd30ce2aac40c7698aa4a1b9520aa649ff2d1c5`
   config = pkgs.writeText "rustc-config" ''
     profile = "compiler" # you may want to choose a different profile, like `library` or `tools`
-    changelog-seen = 2
+    change-id = 115898
 
     [build]
     patch-binaries-for-nix = true