[DOCS] Bugfix newsletter validation

docs/_static/js/newsletter.js

@@ -75,11 +75,11 @@ $(document).ready(function () {
         const emailPattern = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$/;
         if (emailPattern.test(value)) {
             $('#newsletterEmail').removeClass('failed');
-            $('.newsletter-btn').prop('disabled', false);
+            $('.newsletter-submit-btn').prop('disabled', false);
         }
         else {
             $('#newsletterEmail').addClass('failed');
-            $('.newsletter-btn').prop('disabled', true);
+            $('.newsletter-submit-btn').prop('disabled', true);
         }
     }