examples/fetch: update rack version

[orgads]

Version 2.2.2 is vulnerable (CVE-2020-8184).

Types of Changes

• [X] Bug fix.
• [ ] New feature.
• [ ] Performance improvement.

Testing

N/A.

• [ ] I added new tests for my changes.
• [ ] I ran all the tests locally.

[ioquatix]

Please rebase on master.

[orgads]

Done

[ioquatix]

Thanks!