Fix docs code blocks

This is on top of !288 (merged).

---

While checking the docs build artefact from !288 (merged) I noticed the .. code: shell syntax is not only not highlighting the syntax, but also swallowing the text:

This MR turns those code blocks into non-highlighted but working blocks instead (to make them like all the others):

docs/dev/README.rst

@@ -45,9 +45,7 @@ Updatings Pins
 Nixpkgs
 ```````
 
-To update the version of NixOS we deploy with, run:
-
-.. code: shell
+To update the version of NixOS we deploy with, run::
 
    nix-shell --run 'update-nixpkgs'
 
@@ -56,9 +54,7 @@ That will update ``nixpkgs.json`` to the latest release on the nixos release cha
 To update the channel, the script will need to be updated,
 along with the filenames that have the channel in them.
 
-To create a text summary of what an update changes - to put in Merge Requests, for example - run:
-
-.. code: shell
+To create a text summary of what an update changes - to put in Merge Requests, for example - run::
 
     nix-build -A morph -o result-before
     update-nixpkgs
@@ -69,9 +65,7 @@ To create a text summary of what an update changes - to put in Merge Requests, f
 
 Gitlab Repositories
 ```````````````````
-To update the version of packages we import from gitlab, run:
-
-.. code: shell
+To update the version of packages we import from gitlab, run::
 
    nix-shell --command 'update-gitlab-repo nixos/pkgs/<package>/repo.json'