Fix: sha256_bench_{non_recursive,recursive_poseidon2}.sh use default toolchain

[nikurt]

• Remove +nightly from the benchmark scripts.
• Re-flow README.md
• Fix typos in README.md

The code doesn't compile with +nightly due to a dependency crate packed_simd and there is no newer version of that crate.

% cargo +nightly test --release --no-run                                     
   Compiling packed_simd v0.3.9                                                                                       
error[E0557]: feature has been removed                                                                                
   --> /home/nkurtov/.cargo/registry/src/index.crates.io-6f17d22bba15001f/packed_simd-0.3.9/src/lib.rs:218:5
    |                                                                                                                 
218 |     platform_intrinsics,                                                                                        
    |     ^^^^^^^^^^^^^^^^^^^ feature has been removed                                                                
    |                                                                                                                 
    = note: SIMD intrinsics use the regular intrinsics ABI now                                                        
                                                                                                                      
error[E0703]: invalid ABI: found `platform-intrinsic`                                                                 
 --> /home/nkurtov/.cargo/registry/src/index.crates.io-6f17d22bba15001f/packed_simd-0.3.9/src/codegen/llvm.rs:8:8
  |                                                                                                                   
8 | extern "platform-intrinsic" {                                                                                     
  |        ^^^^^^^^^^^^^^^^^^^^ invalid ABI                                                                           
  |                                                                                                                   
  = note: invoke `rustc --print=calling-conventions` for a full list of supported calling conventions.                

[nikurt]

As I can't change reviewers on this PR and many PRs are sitting without getting reviews, I permit myself to CC the users that reviewed recently merged PRs 😁

cc: @mm-zk @aikixd @EmilLuta @shamatar @jules

[EmilLuta]

@robik75 -- I believe you're the right person to look into this.