Remove unavailable accent color buttons from docs [ci skip]

docs/specimen.md

@@ -280,9 +280,6 @@ Click on a tile to change the accent color of the theme:
 <button data-md-color-accent="amber">Amber</button>
 <button data-md-color-accent="orange">Orange</button>
 <button data-md-color-accent="deep-orange">Deep Orange</button>
-<button data-md-color-accent="brown">Brown</button>
-<button data-md-color-accent="grey">Grey</button>
-<button data-md-color-accent="blue-grey">Blue Grey</button>
 
 <script>
   var buttons = document.querySelectorAll("button[data-md-color-accent]");