Fix regression from ac2588c that caused missing translatable strings

We still need the `_p` tag for intltool, so validating the appstream file must be done on the generated file (for now). In the future we should migrate away from intltool which shouldn't be necessary any more (https://wiki.gnome.org/MigratingFromIntltoolToGettext)

xdg/pinta.appdata.xml.in

@@ -6,14 +6,14 @@
   <name>Pinta</name>
   <summary>Edit images and paint digitally</summary>
   <description>
-    <p>
+    <_p>
       Pinta is a image editing, drawing and painting application with a simple
       yet powerful interface.
       Pinta has a wide range of drawing tools, including: freehand, rectangles,
       circles and lines.
       It also has over 35 effects to apply to your images, and also has the
       ability to create unlimited layers to help organize your creativity.
-    </p>
+    </_p>
   </description>
   <launchable type="desktop-id">pinta.desktop</launchable>
   <kudos>