Fix broken link (#280)

Updates reference to publishing.md to fix broken "Publish" button at the top.

docs/schedule/index.md

@@ -5,7 +5,7 @@
 # GTFS Schedule Overview
 
 <div class="landing-page">
-    <a class="button" href="reference">Reference</a><a class="button" href="best-practices">Best Practices</a><a class="button" href="examples">Examples</a><a class="button" href="changes">Changes</a><a class="button" href="validate">Validate</a><a class="button" href="publish">Publish</a>
+    <a class="button" href="reference">Reference</a><a class="button" href="best-practices">Best Practices</a><a class="button" href="examples">Examples</a><a class="button" href="changes">Changes</a><a class="button" href="validate">Validate</a><a class="button" href="publishing">Publish</a>
 </div>
 
 ## Getting Started