[Backport perf-v16] fix(pipeline): config params for predefined write test

[mergify]

Add missed double quote. It was presented by
#9295 and cause to job failure.

Failed run: https://jenkins.scylladb.com/job/scylla-enterprise/job/perf-regression/job/scylla-enterprise-perf-regression-predefined-throughput-steps-write-vnodes/17/

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit 113a0bf)

Parent PR: #9663