Update section heading.

RELNOTES: None.
PiperOrigin-RevId: 178618216
diff --git a/site/_layouts/documentation.html b/site/_layouts/documentation.html
index 11d3c53..8ebd050 100644
--- a/site/_layouts/documentation.html
+++ b/site/_layouts/documentation.html
@@ -169,7 +169,7 @@
              <li><a href="/versions/{{ site.version }}/toolchains.html">Toolchains</a></li>
            </ul>
 
-            <h3>Extend Bazel</h3>
+            <h3>Extending Bazel</h3>
            <ul class="sidebar-nav">
               <li><a href="/versions/{{ site.version }}/skylark/concepts.html">Overview</a></li>