doc: Update "screenshots" in mirror-screenshot-url doc to "media"

Appstream handles remote icon tags too not just screenshots

Author: bbhtt

doc/flatpak-builder.xml

@@ -524,7 +524,7 @@
                 <term><option>--mirror-screenshots-url=URL</option></term>
 
                 <listitem><para>
-                  Mirror any screenshots in the appstream and rewrite the appstream xml
+                  Mirror any media in the appstream and rewrite the appstream xml
                   as if they were on the specified URL. The resulting files will
                   be stored in the "screenshots" subdirectory in the app directory
                   and needs to be copied to the specified URL for the appdata to work.