nixos/manual: normalize <code> -> <literal>

markdown can only translate to <literal>, so normalize to that or to
other more appropriate tags.

nixos/modules/services/backup/borgbackup.xml

@@ -97,7 +97,7 @@ ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAID78zmOyA+5uPG4Ot0hfAy+sLDPU1L4AiIoRYEIVbbQ/
   <para>The following NixOS snippet creates an hourly backup to the service
     (on the host nixos) as created in the section above. We assume
     that you have stored a secret passphrasse in the file
-    <code>/run/keys/borgbackup_passphrase</code>, which should be only
+    <filename>/run/keys/borgbackup_passphrase</filename>, which should be only
     accessible by root
   </para>
   <para>

nixos/modules/services/matrix/synapse.xml

@@ -233,8 +233,8 @@ registration_shared_secret: your-very-secret-secret
    <link xlink:href="https://element.io/blog/welcome-to-element/">Element introductory blog post</link>
    for more information. The following snippet can be optionally added to the code before
    to complete the synapse installation with a web client served at
-   <code>https://element.myhostname.example.org</code> and
-   <code>https://element.example.org</code>. Alternatively, you can use the hosted
+   <literal>https://element.myhostname.example.org</literal> and
+   <literal>https://element.example.org</literal>. Alternatively, you can use the hosted
    copy at <link xlink:href="https://app.element.io/">https://app.element.io/</link>,
    or use other web clients or native client applications. Due to the
    <literal>/.well-known</literal> urls set up done above, many clients should