hugolib: Use the interface value when doing Related search

Currently it makes no practical difference, but this is more a protection if we in the future creates index from the content related fields. That will not work from a shortcode.

See #5071

hugolib/pages_related.go

@@ -73,7 +73,7 @@ func (p Pages) RelatedIndices(doc related.Document, indices ...interface{}) (Pag
 		return nil, err
 	}
 
-	result, err := p.searchDoc(page, indicesStr...)
+	result, err := p.searchDoc(doc, indicesStr...)
 	if err != nil {
 		return nil, err
 	}