tpl: Fix series detection in opengraph

tpl: 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
# in front matter
series:
    - My Series
```

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

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