nixos/manual: Use literalExample when feasible.

Should bring most of the examples into a better consistency regarding
syntactic representation in the manual.

Thanks to @devhell for reporting.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/services/logging/syslog-ng.nix

@@ -49,7 +49,9 @@ in {
       extraModulePaths = mkOption {
         type = types.listOf types.str;
         default = [];
-        example = [ "${pkgs.syslogng_incubator}/lib/syslog-ng" ];
+        example = literalExample ''
+          [ "''${pkgs.syslogng_incubator}/lib/syslog-ng" ]
+        '';
         description = ''
           A list of paths that should be included in syslog-ng's
           <literal>--module-path</literal> option. They should usually