escape module option description

Agorakit uses XML tags in the description string, which isn't valid HTML
and upsets the `vnu` validator we use at the rendering stage.

this is a more general problem: NixOS/nixpkgs#359157

overview/default.nix

@@ -64,7 +64,7 @@
           <table>
             <tr>
               <td>Description:</td>
-              <td>${value.description}</td>
+              <td>${lib.escapeXML value.description}</td>
             </tr>
             <tr>
               <td>Type:</td>