website: "functions" layout "Terraform Language" is back

In the heirarchy, both "Terraform Language" and "Functions" are "up" from
the individual function reference pages, so we'll class them as such to
use the back-facing arrow instead of the forward-facing arrow.

website/layouts/functions.erb

@@ -3,11 +3,11 @@
       <div class="docs-sidebar hidden-print affix-top" role="complementary">
         <ul class="nav docs-sidenav">
           <li<%= sidebar_current("docs-config-index") %>>
-            <a href="/docs/configuration/index.html">Terraform Language</a>
+            <a class="back" href="/docs/configuration/index.html">Terraform Language</a>
           </li>
   
           <li<%= sidebar_current("docs-config-functions") %>>
-            <a href="/docs/configuration/functions.html">Functions</a>
+            <a class="back" href="/docs/configuration/functions.html">Functions</a>
           </li>
   
           <li<%= sidebar_current("docs-funcs-numeric") %>>