Update 12_forms.md : Invalid DOM Property 'class'

225 button has an Invalid DOM property 'class' , react allows 'className'

12_Day_Forms/12_forms.md

@@ -222,7 +222,7 @@ class App extends Component {
             />
           </div>
 
-          <button class='btn btn-success'>Submit</button>
+          <button className='btn btn-success'>Submit</button>
         </form>
       </div>
     )