The key 'actions' is misspelled as actioms.
And the best way to use t() func...

The key 'actions' is misspelled as actioms.

And the best way to use t() function in order to change any string.
$form['actions']['submit']['#value'] = t('Pay for your order');

Please check these two things and the my_module I am assuming is your custom module name otherwise replace it with correct name and clear the cache and test.