Fix name of MCTriggerNum in ANNIEEvent in LoadWCSim.cpp

[ckaragian]

MCTriggerNum is being retrieved as MCTriggernum by other tools. Change back to this

Describe your changes

Checklist before submitting your PR

• This PR implements a single change (one new/modified Tool, or a set of changes to implement one new/modified feature)
• This PR alters the minimum number of files to affect this change
• If this PR includes a new Tool, a README and minimal demonstration ToolChain is provided
• If a new Tool/ToolChain requires model or configuration files, their paths are not hard-coded, and means of generating those files is described in the readme, with examples provided on /pnfs/annie/persistent
• For every new usage, there is a reason the data must be on the heap
• For every new there is a delete, unless I explicitly know why (e.g. ROOT or a BoostStore takes ownership)

Additional Material

Attach any validation or demonstration files here. You may also link to relavant docdb articles.