Simple typo fix in example text (missing #)

To make box three text consistent with box one and two.

Author: blep

learn/values-units/color-hex.html

@@ -57,7 +57,7 @@
 <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>
     </textarea>