Fixed the error string wording

lang/en/question.php

@@ -45,7 +45,7 @@
 $string['cannotmovequestion'] = 'You can\'t use this script to move questions that have files associated with them from different areas.';
 $string['cannotopenforwriting'] = 'Cannot open for writing: {$a}';
 $string['cannotpreview'] = 'You can\'t preview these questions!';
-$string['cannotretrieveqcat'] = 'Could retrieve question category';
+$string['cannotretrieveqcat'] = 'Could not retrieve question category';
 $string['cannotunhidequestion'] = 'Failed to unhide the question.';
 $string['cannotunzip'] = 'Could not unzip file.';
 $string['cannotwriteto'] = 'Cannot write exported questions to {$a}';