replace universal quantifier with existential

Author: cscheid

docs/tools/rstudio.qmd

@@ -17,7 +17,7 @@ To learn more, see the documentation on [Using the Visual Editor](../visual-edit
 
 ## Knitr Engine
 
-Quarto is designed to be highly compatible with existing [R Markdown](https://rmarkdown.rstudio.com/) documents. You should generally be able to use Quarto to render any existing Rmd document without changes.
+Quarto is designed to be highly compatible with existing [R Markdown](https://rmarkdown.rstudio.com/) documents. You should generally be able to use Quarto to render an existing Rmd document without changes.
 
 {{< include _chunk-options.md >}}