Hint and solution mismatch in Async Book Exercise 4: Build a Simple Async Mutex from Scratch

[isSerge]

*Hint*: Use a `tokio::sync::mpsc` channel with capacity 1 as a semaphore.

mpsc channel is suggested, however solution is using actual tokio::sync::Semaphore with capacity 1.

Hint has to be updated to include tokio::sync::Semaphore instead of tokio::sync::mpsc for consistency.