fix: use correct variable names etc. in 'parseFrontMatter'

I wasn't careful enough when rebasing my changes on top of
the Docusaurus v3 upgrade. I've moved the code to the config,
but I forgot to update the variable names and other things.

Author: honzajavorek

docusaurus.config.js

@@ -193,11 +193,11 @@ module.exports = {
         mermaid: true,
         parseFrontMatter: async (params) => {
             const result = await params.defaultParseFrontMatter(params);
-            const isPartial = filePath.split('/').pop()[0] === '_';
+            const isPartial = params.filePath.split('/').pop()[0] === '_';
             if (!isPartial) {
                 const ogImageURL = new URL('https://apify.com/og-image/docs-article');
-                ogImageURL.searchParams.set('title', frontMatter.title);
-                frontMatter.image ??= ogImageURL.toString();
+                ogImageURL.searchParams.set('title', result.frontMatter.title);
+                result.frontMatter.image ??= ogImageURL.toString();
             }
             return result;
         },