Bug fix for ATLASRECTS-8034 in LArDigitalTriggMonAlg

This is a fix for https://its.cern.ch/jira/browse/ATLASRECTS-8034 in the LArDigitalTriggMonAlg which crashed the express processing in the last days (not every run). After consulting with @christos and @ahabouel we think this is a simple typo in a part of the code that seems to be rarely accessed (see JIRA, where this line was introduced). I am marking this as urgent, so that we can hopefully deploy it soon at T0.