Update date.html

When following this [workflow](https://bookdown.org/yihui/blogdown/workflow.html), there is an error when serving the site :

```
Launching the server via the command:
  /Users/antoinesoetewey/Library/Application Support/Hugo/0.82.0/hugo server --bind 127.0.0.1 -p 4321 --themesDir themes -t congo -D -F --navigateToChanged
Error: Error building site: failed to render pages: render of "page" failed: execute of template failed: template: _default/single.html:11:11: executing "main" at <partial "article-meta.html" (dict "context" . "scope" "single")>: error calling partial: execute of template failed: template: partials/article-meta.html:16:36: executing "partials/article-meta.html" at <partial "meta/date.html" .>: error calling partial: "/Users/antoinesoetewey/Documents/GitHub/elsaetantoine/themes/congo/layouts/partials/meta/date.html:2:24": execute of template failed: template: partials/meta/date.html:2:24: executing "partials/meta/date.html" at <.Site.Params.article.dateFormat>: invalid value; expected string
```

By adding these parentheses, the site is served without any error.

Regards,
Antoine

layouts/partials/meta/date.html

@@ -1,10 +1,10 @@
 <time datetime="{{ .Date }}">
-  {{- .Date.Format .Site.Params.article.dateFormat | default "2 January 2006" -}}
+  {{- .Date.Format (.Site.Params.article.dateFormat | default "2 January 2006") -}}
 </time>
 {{ if .Params.showDateUpdated | default (.Site.Params.article.showDateUpdated | default false) }}
   &nbsp;(Updated:&nbsp;
   <time datetime="{{ .Lastmod }}">
-    {{- .Lastmod.Format .Site.Params.article.dateFormat | default "2 January 2006" -}}
+    {{- .Lastmod.Format (.Site.Params.article.dateFormat | default "2 January 2006") -}}
   </time>
   )
 {{ end }}