MDL-39054 quiz statistics: move hard-coded text to lang file

mod/quiz/report/statistics/lang/en/quiz_statistics.php

@@ -69,6 +69,7 @@ Our equation for effective question weight cannot be calculated in this case. Th
 If you edit a quiz and give these question(s) with negative covariance a max grade of zero then the effective question weight of these questions will be zero and the real effective question weight of other questions will be as calculated now.';
 $string['nostudentsingroup'] = 'There are no students in this group yet';
 $string['optiongrade'] = 'Partial credit';
+$string['partofquestion'] = 'Part of question';
 $string['pluginname'] = 'Statistics';
 $string['position'] = 'Position';
 $string['positions'] = 'Position(s)';

mod/quiz/report/statistics/statistics_question_table.php

@@ -74,7 +74,7 @@ class quiz_statistics_question_table extends flexible_table {
 
         if ($responesstats->has_subparts()) {
             $columns[] = 'part';
-            $headers[] = 'Part of question';
+            $headers[] = get_string('partofquestion', 'quiz_statistics');
         }
 
         if ($responesstats->has_response_classes()) {