Correct naming of Devhelp icon in the desktop link

It seems that the naming of the Icon file provided by Devhelp has changed.
We currently add a desktop link via an overlay, which isn't using the
correct name and hence the wrong icon is being used.

Update the icon name to get the right icon.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

overlays/sdk-desktop-icon-devhelp/etc/skel/Desktop/devhelp.desktop

@@ -47,7 +47,7 @@ Keywords[zh_TW]=documentation;information;manual;developer;api;文件;資訊;手
 Exec=devhelp
 Terminal=false
 Type=Application
-Icon=devhelp
+Icon=org.gnome.Devhelp
 Categories=GNOME;GTK;Development;
 StartupNotify=true
 X-GNOME-Bugzilla-Bugzilla=GNOME