MINOR: Replace String literal with existing String variable

[hogimn]

It is minor change.
I found out that String literal is used even though there is existing static String variable.
So, I replaced literal with variable.

Thank you :)

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[divijvaidya]

I see that this is your first change to this repository. Thank you and congratulations 🎉

The change look good, don't mind the failing build, our test suite suite is flaky. As a next step, one of the committers will drop in and merge your change.

[hogimn]

Thanks for your kind reply :)
I am a newbie to Kafka and learning the concept and code through unit tests. I am happy to commit to this repository for the first time 😄.

[chia7712]

LGTM and welcome to kafka :)