Fixed a whitespace typo

htdocs/lang/en.utf8/activity.php

@@ -40,7 +40,7 @@ $string['typeadminmessages'] = 'Administration messages';
 
 $string['type'] = 'Activity type';
 $string['attime'] = 'at';
-$string['prefsdescr'] = 'If you select either of the email options,notifications will still be entered in the Activity log, but they will be automatically marked as read';
+$string['prefsdescr'] = 'If you select either of the email options, notifications will still be entered in the Activity log, but they will be automatically marked as read.';
 
 $string['subject'] = 'Subject';
 $string['date'] = 'Date';