nixpkgs/doc: fix admonition syntax

Match admonition syntax in

https://nixos.org/manual/nixpkgs/unstable/#chap-contributing

Changed files
+4 -4
doc
+3 -3
doc/builders/images/portableservice.section.md
···
The primary tool for interacting with Portable Services is `portablectl`,
and they are managed by the `systemd-portabled` system service.
-
:::{.note}
+
::: {.note}
Portable services are supported starting with systemd 239 (released on 2018-06-22).
:::
···
`units` must be a list of derivations, and their names must be prefixed with the service name (`"demo"` in this case).
Otherwise `systemd-portabled` will ignore them.
-
:::{.Note}
+
::: {.note}
The `.raw` file extension of the image is required by the portable services specification.
:::
···
systemctl enable --now demo.socket
systemctl enable --now demo.service
```
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:::{.Note}
+
::: {.note}
See the [man page](https://www.freedesktop.org/software/systemd/man/portablectl.html) of `portablectl` for more info on its usage.
:::
+1 -1
doc/contributing/coding-conventions.chapter.md
···
in the package expression, attempt build and extract correct hash from error messages.
-
:::{.warning}
+
::: {.warning}
You must use one of these four fake hashes and not some arbitrarily-chosen hash.
See [](#sec-source-hashes-security).