docs: Fix link to the on-change.js example (#884)

Author: stephenwade

docs/react-testing-library/faq.mdx

@@ -11,7 +11,7 @@ to React testing
 <summary>How do I test input onChange handlers?</summary>
 
 TL;DR:
-[Go to the `on-change.js` example](https://codesandbox.io/s/github/kentcdodds/react-testing-library-examples/tree/master/?module=%2Fsrc%2F__tests__%2Fon-change.js&previewwindow=tests)
+[Go to the `on-change.js` example](https://codesandbox.io/s/github/kentcdodds/react-testing-library-examples/tree/main/?module=%2Fsrc%2F__tests__%2Fon-change.js&previewwindow=tests)
 
 In summary: