Fix TOC header (#168)

In English, the translation that is supposed to be shown on top of the table of contents looks like this:

```toml
[whatsInThis]
other = "What's in this {{.Type }}"
```

When it is invoked, there are two mistakes: `{{ i18n "whatsInThis" humanize .Type }}`.

The first mistake is that you cannot call `humanize` in this context. The more severe one is that if you remove `humanize` it will pass the `.Type` directly to the i18n and that tries to extract `.Type` from it.

layouts/partials/menu-contextual.html

@@ -5,7 +5,7 @@
 
 {{- if .Params.toc -}}
   <div class="bg-light-gray pa3 nested-list-reset nested-copy-line-height nested-links">
-    <p class="f5 b mb3">{{ i18n "whatsInThis" humanize .Type }}</p>
+    <p class="f5 b mb3">{{ i18n "whatsInThis" . }}</p>
       {{ .TableOfContents }}
   </div>
 {{- end -}}