doc: fix callout in README

README.md

@@ -5,11 +5,7 @@
 [![R build status](https://github.com/juba/rmdformats/workflows/R-CMD-check/badge.svg)](https://github.com/juba/rmdformats/actions?query=workflow%3AR-CMD-check)
 
 
-{% note %}
-
-**Note for quarto users:** these templates are only suitable for RMarkdown documents. I published one quarto custom format called [bookup](https://github.com/juba/bookup-html) which is a porting of the `robobook` template.
-
-{% endnote %}
+> **Note for quarto users:** these templates are only suitable for RMarkdown documents. I published one quarto custom format called [bookup](https://github.com/juba/bookup-html) which is a porting of the `robobook` template.
 
 This R package provides ready-to-use HTML output formats and templates for
 RMarkdown documents. The goal is to produce clean documents "out of the box",