scripts/nvmf_perf: add num_jobs to configuration file

"num_jobs" is described as required parameter in README.md
but not included in example configuration file.

Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Change-Id: I1077d027d6c7dbee49afa1df4345b4dd478b5ad6
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/13615


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Reviewed-by: Michal Berger <michal.berger@intel.com>