docs(adapters): remove html_to_markdown post-processor from selectors reference (#1253)

Author: gildesmarais

src/content/docs/ruby-gem/reference/selectors.mdx

@@ -242,7 +242,6 @@ Extractors define how to get the value from a selected element.
 Post-processors manipulate the extracted value.
 
 - `gsub`: Performs a global substitution on a string.
-- `html_to_markdown`: Converts HTML to Markdown.
 - `markdown_to_html`: Converts Markdown to HTML.
 - `parse_time`: Parses a string into a `Time` object.
 - `parse_uri`: Resolves a relative URL against `channel.url` and returns the normalized URL string.