CONTRIBUTING.md is outdated for installing emacs

[The section](https://github.com/IntersectMBO/formal-ledger-specifications/blob/f4eed8bfcbe96a0bda51375b85b6012da9ec159a/CONTRIBUTING.md#emacs) on installing emacs asks to run `nix-shell`, which at the moment fails with:

```
$ nix-shell
error: nix-shell requires a single derivation
Try 'nix-shell --help' for more information.
```

I got passed this by
```
--- a/default.nix
+++ b/default.nix
@@ -290,6 +290,8 @@ let
inherit (locales) LANG LC_ALL LOCALE_ARCHIVE;
packages = [
agdaWithPackages # Agda 2.7.0.1 + all pinned libraries
+ haskellPackages.Agda
+ emacs
+ dejavu_fonts
+ fontconfig
fls-shake # Build system for generating outputs
python311 # Python for basic scripting
hpack # Haskell package helper
```
then
```
nix develop
agda-mode setup
emacs src/Ledger.hs
```

Either the instructions need an update, or the nix configuration needs to be fixed.