Update comment in TPCBarycenterMatch

The comment in `angle` of [TPCBarycenterMatch](https://github.com/SBNSoftware/sbnobj/blob/cb16afd4f78e3baee7f36ce730673e89d5e5878a/sbnobj/Common/Reco/TPCPMTBarycenterMatch.h#L50) shows an angle in `us` and it should be corrected.