nixos/*: use properly indented strings for option docs

using regular strings works well for docbook because docbook is not as
whitespace-sensitive as markdown. markdown would render all of these as
code blocks when given the chance.

nixos/modules/services/databases/mongodb.nix

@@ -35,9 +35,9 @@ in
         default = pkgs.mongodb;
         defaultText = literalExpression "pkgs.mongodb";
         type = types.package;
-        description = "
+        description = ''
           Which MongoDB derivation to use.
-        ";
+        '';
       };
 
       user = mkOption {