Stop highlighting --help output in README as shell (#1655)

This PR is meant to address the following obviously unintended GitHub
rendering:

![image](https://user-images.githubusercontent.com/73739153/210713719-7fb465b1-db91-4074-8a0c-4efa3c47c2f4.png)

ruff_dev/src/generate_cli_help.rs

@@ -28,7 +28,7 @@ pub fn main(cli: &Cli) -> Result<()> {
         print!("{output}");
     } else {
         replace_readme_section(
-            &format!("```shell\n{output}\n```\n"),
+            &format!("```\n{output}\n```\n"),
             HELP_BEGIN_PRAGMA,
             HELP_END_PRAGMA,
         )?;