Added missing plural string for elements (#607)

lang/en/customcert.php

@@ -195,6 +195,7 @@
 $string['showposxy_desc'] = 'This will show the X and Y position when editing of an element, allowing the user to accurately specify the location.
 
 This isn\'t required if you plan on solely using the drag and drop interface for this purpose.';
+$string['subplugintype_customcertelement_plural'] = 'Elements';
 $string['taskemailcertificate'] = 'Handles emailing certificates.';
 $string['templatename'] = 'Template name';
 $string['templatenameexists'] = 'That template name is currently in use, please choose another.';