MDL-29108 Fixing a typo

Spotted by Sam during the review (23)

lang/en/grading.php

@@ -38,7 +38,7 @@
 $string['gradingmethod'] = 'Grading method';
 $string['gradingmethod_help'] = 'Choose the advanced grading method that should be used for calculating grades in the given context.
 
-To disable advance grading and switch back to the default grading mechanism, choose \'Simple direct grading\'.';
+To disable advanced grading and switch back to the default grading mechanism, choose \'Simple direct grading\'.';
 $string['gradingmethodnone'] = 'Simple direct grading';
 $string['gradingmethods'] = 'Grading methods';
 $string['manageactionclone'] = 'Create new grading form from a template';