Fix a syntax error in leaking.md

[BillXWB]

guards should be mutable here:

nomicon/src/leaking.md

Lines 208 to 232 in 9493715

	<!-- ignore: simplified code -->
	```rust,ignore
	let mut data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
	{
	let guards = vec![];
	for x in &mut data {
	// Move the mutable reference into the closure, and execute
	// it on a different thread. The closure has a lifetime bound
	// by the lifetime of the mutable reference `x` we store in it.
	// The guard that is returned is in turn assigned the lifetime
	// of the closure, so it also mutably borrows `data` as `x` did.
	// This means we cannot access `data` until the guard goes away.
	let guard = thread::scoped(move || {
	*x *= 2;
	});
	// store the thread's guard for later
	guards.push(guard);
	}
	// All guards are dropped here, forcing the threads to join
	// (this thread blocks here until the others terminate).
	// Once the threads join, the borrow expires and the data becomes
	// accessible again in this thread.
	}
	// data is definitely mutated here.
	```

[ehuss]

Thanks!