docs: correct example in MissedTickBehavior::Burst

[ekzhang]

Motivation

Hi there! While looking at the docs for MissedTickBehavior, I noticed that the comments in the code example have a very small error.

use tokio::time::{interval, Duration};

let mut interval = interval(Duration::from_millis(50));

task_that_takes_200_millis().await;
// The `Interval` has missed a tick

// Since we have exceeded our timeout, this will resolve immediately
interval.tick().await;

// Since we are more than 100ms after the start of `interval`, this will
// also resolve immediately.
interval.tick().await;

// Also resolves immediately, because it was supposed to resolve at
// 150ms after the start of `interval`
interval.tick().await;

// Resolves immediately
interval.tick().await;

// Since we have gotten to 200ms after the start of `interval`, this
// will resolve after 50ms
interval.tick().await;

The code example forgets that the first interval.tick() resolves immediately after creation. So if you print out all of the elapsed times after each tick, you'll see that they all resolve immediately.

Here's a playground link that shows the issue. If you run this code then you'll see the following output:

1. 201.484664ms
2. 201.523575ms
3. 201.528885ms
4. 201.535065ms
5. 201.555235ms

However, if you uncomment line 11, which has the extra interval.tick().await; that this PR proposes, then the output changes to what would be expected in the docs:

1. 202.916324ms
2. 203.013206ms
3. 203.050227ms
4. 203.094128ms
5. 251.418813ms

Solution

This PR fixes the issue by just adding one more line with interval.tick().await at the beginning of the example, before the long task.

[Darksonn]

Thanks.