[i18n]KOSync: use already translated "Not available" instead of "Unavailable" (#10648)

Cf. https://github.com/koreader/koreader/pull/10605#discussion_r1252639047

plugins/kosync.koplugin/main.lua

@@ -94,7 +94,7 @@ end
 
 function KOSync:getSyncPeriod()
     if not self.settings.auto_sync then
-        return _("Unavailable")
+        return _("Not available")
     end
 
     local period = self.settings.pages_before_update