fix detectorList argument for Pb-Pb

[f3sch]

In #2128, I introduced a bug where the detectorList argument did not have a separating space after it.
This leads to an incorrect construction of the cmd string --detectorList ALICE2--configKeyValues for the qed background sim. I only tested my changes in pp not in Pb-Pb...sorry

[github-actions]

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[sawenzel]

No worries. We should improve the CI and test various different workflows.