Fix social media preview url tiddler name (#8496)

The control panel name and the name in the template don't match. I
figured the shorter name is better, but let me know if you like the
longer name.

This is a small fix for recently merged PR #8441.

core/templates/social-metadata.tid

@@ -27,12 +27,12 @@ tags: $:/tags/RawMarkupWikified
 <<meta-plain "og:type" "website" "property">>
 <<meta-wikified "og:title" "$:/SiteTitle" "property">>
 <<meta-wikified "og:description" "$:/SiteSubtitle" "property">>
-<<meta-plain "og:image" "$:/SitePreviewImageUrl" "property">>
+<<meta-plain "og:image" "$:/SitePreviewUrl" "property">>
 
 <!-- Twitter Meta Tags -->
 <<meta-plain "twitter:card" "summary_large_image">>
 <<meta-plain "twitter:domain" "$:/SiteDomain" "property">>
 <<meta-plain "twitter:url" "$:/SiteUrl" "property">>
 <<meta-wikified "twitter:title" "$:/SiteTitle">>
 <<meta-wikified "twitter:description" "$:/SiteSubtitle">>
-<<meta-plain "twitter:image" "$:/SitePreviewImageUrl">>
+<<meta-plain "twitter:image" "$:/SitePreviewUrl">>