Minor: Documentation of the default prover/verifier example causes issue in rust analyser

[radicleart]

PR fixes a minor issue in the documentation of the default prover/verifier example - note the cargo build is independent of this problem.

The prover example causes rust analyser (vs code) to flag an error - requiring a ref to be added here E::from(42u32) - adding the ref causes the analyser to flag another error asking for the ref to be removed.

let current_state = &frame.current()[0];
let next_state = current_state.exp(3u32.into()) + E::from(42u32); 

Removing the ref in the first line solves the problem.

[irakliyk]

Looks good! Thank you for fixing it!