Correct mistyping of language string in previous commit.

lang/en/qtype_stack.php

@@ -265,7 +265,7 @@ $string['optionsnotrequired'] = 'This input type does not require any options.';
 $string['penalty'] = 'Penalty';
 $string['penalty_help'] = 'The penalty scheme deducts this value from the result of each PRT for each different and valid attempt which is not completely correct.';
 $string['penalty_link'] = '%%WWWROOT%%/question/type/stack/doc/doc.php/Authoring/Feedback.md';
-$string['penaltyf'] = 'The penalty must be a numeric value between 0 and 1 or a variable (which is not checked).';
+$string['penaltyerror'] = 'The penalty must be a numeric value between 0 and 1 or a variable (which is not checked).';
 $string['penaltyerror2'] = 'The penalty must empty, or be a numeric value between 0 and 1.';
 $string['prtcorrectfeedback'] = 'Standard feedback for correct';
 $string['prtheading'] = 'Potential response tree: {$a}';