Fix OpenGraph modified_time to use .Params.lastmod

[versha890]

This PR fixes the issue with the OpenGraph article:modified_time meta tag. Previously, regenerating the site updated the modification time incorrectly for all posts. Now, it uses .Params.lastmod to reflect the actual last modified date set in the post front matter.

##Changes

Updated _partials/opengraph.html: {{ with .Params.lastmod }} <meta property="article:modified_time" content="{{ . | time.Format "2006-01-02T15:04:05Z07:00" }}" /> {{ end }}