Adds a comment to Interval.tick() example

[clintfred]

The comment simply reminds the reader that the first tick of an Interval completes immediately.

[Darksonn]

Thanks.

[Darksonn]

You'll need to fix the trailing whitespace.

Diff in /home/runner/work/tokio/tokio/tokio/src/time/interval.rs at line 422:
     /// async fn main() {
     ///     let mut interval = time::interval(Duration::from_millis(10));
     ///
-    ///     interval.tick().await; 
+    ///     interval.tick().await;
     ///     // approximately 0ms have elapsed. The first tick completes immediately.
-    ///     interval.tick().await; 
+    ///     interval.tick().await;
     ///     interval.tick().await;
     ///
     ///     // approximately 20ms have elapsed.
Diff in /home/runner/work/tokio/tokio/tokio/src/time/interval.rs at line 422:
     /// async fn main() {
     ///     let mut interval = time::interval(Duration::from_millis(10));
     ///
-    ///     interval.tick().await; 
+    ///     interval.tick().await;
     ///     // approximately 0ms have elapsed. The first tick completes immediately.
-    ///     interval.tick().await; 
+    ///     interval.tick().await;
     ///     interval.tick().await;
     ///
     ///     // approximately 20ms have elapsed.
Please run `rustfmt --edition 2018 $(git ls-files '*.rs')` to fix rustfmt errors.
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