PMGTruthWeightTool fix incorrect FileMetaData container access

Following recent PATHelp email discussion:
https://groups.cern.ch/group/hn-atlas-PATHelp/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DPATHelp%2FLists%2FArchive%2FMC%20channel%20not%20found%20in%20PMGTruthWeightTool&FolderCTID=0x0120020084D7E80CB0C3394A8D54EF07C309B044

Currently encountering runtime warnings:

PMGTruthWeightTool     INFO    Attempting to retrieve truth meta data from a new file...
PMGTruthWeightTool     INFO    Attempting to retrieve MC channel number...
PMGTruthWeightTool     WARNING ... MC channel number could not be loaded
PMGTruthWeightTool     INFO    Loaded xAOD::TruthMetaDataContainer
PMGTruthWeightTool     WARNING MC channel number in TruthMetaData is invalid - assuming that channel 0 has the correct information.
PMGTruthWeightTool     INFO    Attempting to load weight meta data from xAOD TruthMetaData for channel 0
PMGTruthWeightTool     INFO    Successfully loaded information about 217 weights
PMGTruthWeightTool     INFO    Attempting to load weight meta data from xAOD TruthMetaData for channel 0
PMGTruthWeightTool     INFO    Successfully loaded information about 217 weights

While the MC DSID may be appropriately captured by later fallback procedures in the tool, and should continue to work with correct number of weights, the warnings still exist because simply it is trying to access FileMetaData info from the wrong container, hence failing to find the DSID at first attempt. This commit fixes this, which should suppress the warnings.

Pinging @lcorpe @kolehman