docs: update documentation

Author: mrholek

docs/content/migration.md

@@ -392,8 +392,8 @@ toc: true
 - **All plugins can now accept a CSS selector as the first argument.** You can either pass a DOM element or any valid CSS selector to create a new instance of the plugin:
 
   ```js
-  var modal = new coreui.Modal('#myModal')
-  var dropdown = new coreui.Dropdown('[data-coreui-toggle="dropdown"]')
+  const modal = new coreui.Modal('#myModal')
+  const dropdown = new coreui.Dropdown('[data-coreui-toggle="dropdown"]')
   ```
 
 - `popperConfig` can be passed as a function that accepts the Bootstrap's default Popper config as an argument, so that you can merge this default configuration in your way. **Applies to dropdowns, popovers, and tooltips.**

docs/static/assets/js/validate-forms.js

@@ -3,12 +3,12 @@
   'use strict'
 
   // Fetch all the forms we want to apply custom Bootstrap validation styles to
-  var forms = document.querySelectorAll('.needs-validation')
+  const forms = document.querySelectorAll('.needs-validation')
 
   // Loop over them and prevent submission
   Array.prototype.slice.call(forms)
-    .forEach(function (form) {
-      form.addEventListener('submit', function (event) {
+    .forEach(form => {
+      form.addEventListener('submit', event => {
         if (!form.checkValidity()) {
           event.preventDefault()
           event.stopPropagation()