Update form label with variant title

gistfile1.js

$(function() {
  $(document).on('change', '.single-option-selector', function() { 

 	  var label = $('.notify_form label[for^=contact]');

    if (label.is(':visible')) {
      var title = $('.single-option-selector').map(function() { return $(this).val() }).get().join(' ');
      label.text("Notify me when the " + title + " are in stock");
    }
  });
});

Add this code to the snippets/product-notify-me.liquid just before the closing </script> tag.