Issue #3221197: add equation to captcha label for accessibility

Closes #3221197

captcha.module

@@ -667,9 +667,8 @@ function captcha_captcha($op, $captcha_type = '') {
         // or string overrides.
         $result['form']['captcha_response'] = [
           '#type' => 'textfield',
-          '#title' => t('Math question'),
+          '#title' => t('Math question (@x + @y =)', ['@x' => $x, '@y' => $y]),
           '#description' => t('Solve this simple math problem and enter the result. E.g. for 1+3, enter 4.'),
-          '#field_prefix' => t('@x + @y =', ['@x' => $x, '@y' => $y]),
           '#size' => 4,
           '#maxlength' => 2,
           '#required' => TRUE,