Fixed broken link

Currently, the Previous link throws a 404.

I have updated the link from (http://tachyons.io/docs/layout/debug/) to correct URL (http://tachyons.io/docs/debug/)

Author: desiredpersona

docs/debug-grid/index.html

@@ -181,7 +181,7 @@ <h1>Image Title</h1>
         <div class="mt5 cf">
           <div class="dib mr4">
             <h4 class="f6 fw6">Previous</h4>
-            <a href="/docs/layout/debug/" class="link fw6 blue dim">Debug</a>
+            <a href="/docs/debug/" class="link fw6 blue dim">Debug</a>
           </div>
           <div class="dib">
             <h4 class="f6 fw6">Next</h4>