Update recipe to include file content in a chunk

[cderv]

The recipe is this one https://bookdown.org/yihui/rmarkdown-cookbook/option-code.html

When available in next version of knitr following https://github.com/yihui/knitr/commit/545aed32274955be2109150ce1ef4e24fe3732de

```{r, code=xfun::read_utf8('your-script.R')}

could be written

````markdown
```{r, file='your-script.R'}

This also relates to #360 as it will impact the syntax

[yihui]

I just updated this section. But the cairoDevice package has been archived on CRAN, so GHA failed: https://github.com/yihui/rmarkdown-cookbook/runs/4723294557?check_suite_focus=true (Update: I just removed the soft dependency on cairoDevice).

[cderv]

Thank you !