Move second image to it's own screenshot tag

The screenshot tag shouldn't contain more than one image

data/appdata.xml

@@ -11,6 +11,8 @@
     <screenshots>
         <screenshot type="default">
             <image>https://kx.studio/screenshots/carla.png</image>
+        </screenshot>
+        <screenshot>
             <image>https://kx.studio/screenshots/carla-patchbay.png</image>
         </screenshot>
     </screenshots>