Update color-hex.html

Added "#" in box three div, matching the other two examples.

Author: vgiffin

learn/values-units/color-hex.html

@@ -35,7 +35,7 @@
       <div class="wrapper">
         <div class="box one">#02798b</div>
         <div class="box two">#c55da1</div>
-        <div class="box three">128a7d</div>
+        <div class="box three">#128a7d</div>
       </div>
     </section>