PowhegControl: bugfix in MadSpin log messages which was triggering an error by mistake - AGENE-2086
requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:21.6-Powheg_MadSpin_ErrorLog_Fix into 21.6
I'm fixing a bug in the use of MadSpin in PowhegControl. The message "generating the production square matrix element" was creating an ERROR.