templates: Fix series detection in opengraph

When inside front matter you specified series with spaces,
then the opengraph template wouldn't detect other articles,
because in `.Site.Taxonomies.series` they are stored by
urlized key.

Example:

```yaml
series:
    - My Series
```

```gohtml
{{/* in a template */}}
{{- $series := index .Site.Taxonomies.series$name }}

{{/* was resolved to */}}
{{- $series := index {'my-series': ...} "MySeries" }}
```

(cherry picked from commit d2d493ab5d6a054001a8448ea0de2949dac4b30e)

layouts/partials/templates/opengraph.html

@@ -46,7 +46,7 @@
 {{- $permalink := .Permalink }}
 {{- $siteSeries := .Site.Taxonomies.series }}{{ with .Params.series }}
 {{- range $name := . }}
-  {{- $series := index $siteSeries $name }}
+  {{- $series := index $siteSeries ($name | urlize) }}
   {{- range $page := first 6 $series.Pages }}
     {{- if ne $page.Permalink $permalink }}<meta property="og:see_also" content="{{ $page.Permalink }}" />{{ end }}
   {{- end }}