nixos/*: turn inline code blocks into more appropriate things

nixos/modules/config/zram.nix

@@ -108,7 +108,7 @@ in
           but is slow. <literal>lz4</literal> has bad compression, but is fast.
           <literal>zstd</literal> is both good compression and fast, but requires newer kernel.
           You can check what other algorithms are supported by your zram device with
-          <programlisting>cat /sys/class/block/zram*/comp_algorithm</programlisting>
+          <command>cat /sys/class/block/zram*/comp_algorithm</command>
         '';
       };
     };

nixos/modules/services/monitoring/mackerel-agent.nix

@@ -24,7 +24,7 @@ in {
         Path to file containing the Mackerel API key. The file should contain a
         single line of the following form:
 
-        <literallayout>apikey = "EXAMPLE_API_KEY"</literallayout>
+        <literal>apikey = "EXAMPLE_API_KEY"</literal>
       '';
     };
 

nixos/modules/services/web-apps/nextcloud.nix

@@ -572,7 +572,7 @@ in {
       description = ''
         Secret options which will be appended to nextcloud's config.php file (written as JSON, in the same
         form as the <xref linkend="opt-services.nextcloud.extraOptions"/> option), for example
-        <programlisting>{"redis":{"password":"secret"}}</programlisting>.
+        <literal>{"redis":{"password":"secret"}}</literal>.
       '';
     };