Fix horizontal divider on light (#1997)

## Checklist

- [ ] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [ ] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [ ] I have not broken the cheatsheet

Author: pokey

packages/cursorless-org/src/components/Layout.tsx

@@ -27,7 +27,7 @@ const components: MDXComponents = {
       {children}
     </h4>
   ),
-  hr: () => <hr className="my-8 border-white" />,
+  hr: () => <hr className="my-8 border-black dark:border-white" />,
   ul: ({ children }) => <ul className="list-disc ml-8">{children}</ul>,
   ol: ({ children }) => <ol className="list-decimal ml-8">{children}</ol>,
   li: ({ children }) => <li className="my-2">{children}</li>,