Fixed some missing strings

lang/en_utf8/grades.php

@@ -130,7 +130,8 @@
 $string['excludedhelp'] = 'If -excluded- is switched on, this grade will be excluded from any aggregation performed by any parent grade item or category.';
 $string['expand'] = 'Expand Category';
 $string['export'] = 'Export';
-$string['exportletters'] = 'Export letters';
+$string['exportfeedback'] = 'Include feedback in export';
+$string['exportletters'] = 'Export letter grades as letters';
 $string['exportlettershelp'] = 'For grade items that use grade letters instead of values, the letters will be exported instead of the actual values.';
 $string['exportplugins'] = 'Export plugins';
 $string['extracredit'] = 'Extra Credit';