Fix printable cheatsheet link (#814)

The printable cheatsheet link seems to be broken since gitlab renamed `master` branch to `main`

Author: scoobster17

docs/react-testing-library/cheatsheet.mdx

@@ -170,4 +170,4 @@ getByText((content, element) => content.startsWith('Hello'))
 **[Get the printable cheat sheet][cheatsheet]**
 
 [cheatsheet]:
-  https://github.com/testing-library/react-testing-library/raw/master/other/cheat-sheet.pdf
+  https://github.com/testing-library/react-testing-library/raw/main/other/cheat-sheet.pdf