metainfo: Remove final dots in captions

Flathub (aka `appstream-cli validate`) doesn't like them.

data/org.gnome.Fractal.metainfo.xml.in.in

@@ -21,15 +21,15 @@
   <screenshots>
     <screenshot type="default">
       <image type="source">https://gitlab.gnome.org/GNOME/fractal/raw/main/screenshots/main.png</image>
-      <caption>Fractal’s main window.</caption>
+      <caption>Fractal’s main window</caption>
     </screenshot>
     <screenshot>
       <image type="source">https://gitlab.gnome.org/GNOME/fractal/raw/main/screenshots/media-history.png</image>
-      <caption>View the media history of a Matrix room.</caption>
+      <caption>View the media history of a Matrix room</caption>
     </screenshot>
     <screenshot>
       <image type="source">https://gitlab.gnome.org/GNOME/fractal/raw/main/screenshots/adaptive.png</image>
-      <caption>Fractal’s interface adapts to small screens.</caption>
+      <caption>Fractal’s interface adapts to small screens</caption>
     </screenshot>
   </screenshots>