Add class to the Skip Link in the accessibility.md (vuejs#2123)

- CSS syntax is case-insensitive so it is better to use kebab case (skip-link) instead of camel case (skipLink)
- The anchor tag must have a class of skip-link

Author: orbanszlrd

src/guide/best-practices/accessibility.md

@@ -15,15 +15,15 @@ Typically this is done on the top of `App.vue` as it will be the first focusable
 ```vue-html
 <ul class="skip-links">
   <li>
-    <a href="#main" ref="skipLink">Skip to main content</a>
+    <a href="#main" ref="skipLink" class="skip-link">Skip to main content</a>
   </li>
 </ul>
 ```
 
 To hide the link unless it is focused, you can add the following style:
 
 ```css
-.skipLink {
+.skip-link {
   white-space: nowrap;
   margin: 1em auto;
   top: 0;
@@ -32,7 +32,7 @@ To hide the link unless it is focused, you can add the following style:
   margin-left: -72px;
   opacity: 0;
 }
-.skipLink:focus {
+.skip-link:focus {
   opacity: 1;
   background-color: white;
   padding: 0.5em;