nixos/xserver: refactor/fix xml tags for keyboard layout overrides

With the current XML tags there is an issue when building with a custom keyboard.
The description of the new keyboard layout will be missing in the built file: xkeyboard-config-2.27/share/X11/xkb/rules/evdev.lst

pkgs/servers/x11/xorg/overrides.nix

@@ -468,7 +468,7 @@ self: super:
         <model>
           <configItem>
             <name>${name}</name>
-            <_description>${layout.description}</_description>
+            <description>${layout.description}</description>
             <vendor>${layout.description}</vendor>
           </configItem>
         </model>
@@ -484,8 +484,8 @@ self: super:
         <layout>
           <configItem>
             <name>${name}</name>
-            <_shortDescription>${name}</_shortDescription>
-            <_description>${layout.description}</_description>
+            <shortDescription>${name}</shortDescription>
+            <description>${layout.description}</description>
             <languageList>
               ${concatMapStrings (lang: "<iso639Id>${lang}</iso639Id>\n") layout.languages}
             </languageList>