update F-Droid DAVdroid link (#1253)

the information about the invalid link comes from here -> https://discourse.mailinabox.email/t/admin-sync-guide-contacts-and-calendar-davdroid-3-69-free-here/2528

management/templates/sync-guide.html

@@ -30,7 +30,7 @@
 
 			<table class="table">
 			<thead><tr><th>For...</th> <th>Use...</th></tr></thead>
-			<tr><td>Contacts and Calendar</td> <td><a href="https://play.google.com/store/apps/details?id=at.bitfire.davdroid">DAVdroid</a> ($3.69; free <a href="https://f-droid.org/repository/browse/?fdfilter=dav&fdid=at.bitfire.davdroid">here</a>)</td></tr>
+			<tr><td>Contacts and Calendar</td> <td><a href="https://play.google.com/store/apps/details?id=at.bitfire.davdroid">DAVdroid</a> ($3.69; free <a href="https://f-droid.org/packages/at.bitfire.davdroid/">here</a>)</td></tr>
 			<tr><td>Only Contacts</td> <td><a href="https://play.google.com/store/apps/details?id=org.dmfs.carddav.sync">CardDAV-Sync free beta</a> (free)</td></tr>
 			<tr><td>Only Calendar</td> <td><a href="https://play.google.com/store/apps/details?id=org.dmfs.caldav.lib">CalDAV-Sync</a> ($2.89)</td></tr>
 			</table>