docs(tutorial/step-12): replaced a missing apostrophe and fixed grammar errors
docs(tutorial/step-12): replaced a missing apostrophe and fixed grammar errors

Fixed a missing apostrophe and some grammar in the Animating 'ngClass' with JavaScript section

ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js