2018-04-11-making-a-cockpit-application.md: Update cockpit metainfo

At some point, cockpit changed the application id and this page linked from
documentation includes an outdated example. If used, this results in AppStream
metadata for the app being dropped as they don't belong to any app.

_posts/2018-04-11-making-a-cockpit-application.md

@@ -28,7 +28,7 @@ page.
 For a Cockpit component, the AppStream data looks like this:
 ```xml
 <component type="addon">
-  <id>org.cockpit-project.demo-app</id>
+  <id>org.cockpit_project.demo-app</id>
   <metadata_license>CC0-1.0</metadata_license>
   <name>Demo Application</name>
   <summary>
@@ -37,13 +37,13 @@ For a Cockpit component, the AppStream data looks like this:
   <description>
     <p>This is a demo application</p>
   </description>
-  <extends>cockpit.desktop</extends>
+  <extends>org.cockpit_project.cockpit</extends>
   <launchable type="cockpit-manifest">demo-app</launchable>
 </component>
 ```
 
 This would be placed at
-`/usr/share/metainfo/org.cockpit-project.demo-app.metainfo.xml` in
+`/usr/share/metainfo/org.cockpit_project.demo-app.metainfo.xml` in
 your package.
 
 The important bit is the `launchable` element with type
@@ -53,7 +53,7 @@ package](https://cockpit-project.org/guide/latest/packages.html) for
 your page.
 
 Use the component type `addon` and add an `extends` element for
-`cockpit.desktop`.
+`org.cockpit_project.cockpit`.
 
 You should of course come up with your own value for the `id` element,
 in the usual reverse-DNS style, and adjust the filename accordingly.