appstream: Fix screenshot

The one we were pointing to is 404, so got some screenshots from the
documentation.

linux/org.qgis.qgis.appdata.xml.in

@@ -4,7 +4,9 @@
   <name>QGIS Desktop</name>
   <summary>A Geographic Information System</summary>
   <screenshots>
-    <screenshot type="default"><caption>QGIS</caption><image type="source">https://qgis.org/en/_static/images/about-screenshot.png</image></screenshot>
+    <screenshot type="default"><caption>QGIS</caption><image type="source">https://docs.qgis.org/testing/en/_images/answer_symbology2.png</image></screenshot>
+    <screenshot type="default"><caption>3D Map View</caption><image type="source">https://docs.qgis.org/testing/en/_images/3dmapview.png</image></screenshot>
+    <screenshot type="default"><caption>QGIS Project Properties</caption><image type="source">https://docs.qgis.org/testing/en/_images/selectCRS.png</image></screenshot>
   </screenshots>
   <developer id="qgis.org"><name>QGIS Development Team</name></developer>
   <description><p>QGIS is a user friendly Open Source Geographic Information System (GIS) licensed under the GNU General Public License. QGIS is an official project of the Open Source Geospatial Foundation (OSGeo). It runs on Linux, Unix, Mac OSX, Windows and Android and supports numerous vector, raster, and database formats and functionalities.</p></description>