html: add href to checkpoint's git

html/index.html

@@ -224,7 +224,7 @@
                 Difficulty {{ tl:text challengeDifficulty }}, elapsed 0ms, 0h, 0h/s
             </p>
 
-            <a class="bottom-credit-text">
+            <a class="bottom-credit-text" href="https://github.com/vaxerski/checkpoint">
                 <i>Powered by checkpoint v{{ tl:text checkpointVersion }}</i>
             </a>
         </div>