fix Fud link
Calyx uses an array of tools to simplify and streamline compiler testing,
development, and usage.
-- [Fud: The Calyx Driver](./fud.md): Invoke various tools to compile, test, and simulation input programs.
+- [Fud: The Calyx Driver](../fud): Invoke various tools to compile, test, and simulation input programs.
- [Runt: Run Tests](./runt.md): Parallel expectation testing framework.
