Remove the link-button focus css from the anchor-is-valid doc

Author: jessebeach

docs/rules/anchor-is-valid.md

@@ -153,10 +153,6 @@ Then use styling to change its appearance to that of a link:
   margin: 0;
   padding: 0;
 }
-
-.link-button:focus {
-  text-decoration: thin dotted;
-}
 ```
 
 This button element can now also be used inline in text.