Minor: (Doc) Enable rt-multi-thread feature for sample code

[hsiang-c]

Which issue does this PR close?

Closes #.

Rationale for this change

• The sample code in docs/source/user-guide/example-usage.md is not working with tokio = "1.0" dependency. The error I got is

error: The default runtime flavor is `multi_thread`, but the `rt-multi-thread` feature is disabled.
 --> src/main.rs:3:1
  |
3 | #[tokio::main]
  | ^^^^^^^^^^^^^^
  |
  = note: this error originates in the attribute macro `tokio::main` (in Nightly builds, run with -Z macro-backtrace for more info)

What changes are included in this PR?

• Enabled rt-multi-thread feature in tokio.

Are these changes tested?

Yes. I got the expected output

+---+----------------+
| a | MIN(example.b) |
+---+----------------+
| 1 | 2              |
+---+----------------+

Are there any user-facing changes?

Nope

[alamb]

Thanks you @hsiang-c 🙏 -- very nice

[hsiang-c]

Thank you @alamb