Update to next stable version to avoid cargo-doc ICE

[CPerezz]

I noticed we've changed our toolchain file to use 1.56.1. I belive @liangcc needed it. See: #157

There's an ICE reported there with cargo doc which basically won't allow any of our CI runs to pass. See this run where we get reported a panic from the compiler in our CI: https://github.com/appliedzkp/zkevm-circuits/runs/4231504955?check_suite_focus=true#step:5:425

This is solved in nightly and when the new stable is out (the fix is now in beta).

See: rust-lang/rust#84738 for more details of the issue