Fix typo in histogram name.

Closed Mark Hodgkinson requested to merge mhodgkin/athena:FixPhysValHistName_PFO_Dec18 into 21.0

Fixes typo in histogram name needed for physics validation. No effect on Tier0 because the code only runs in physics validation.

Putting into 21.0 such that it propagates everywhere else by default (including 21.2 which needs it immediately). Tagging @khoo , @tcuhadar ,@mann ,@sboutle

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