Fix double inversion of MET covariance matrix

Bug was inserted during https://github.com/cms-sw/cmssw/pull/5417,

MET covariance matrix was inverted twice instead of once, now corrected. Impacts MET significance results only.
Old code but this should be std::abs() .
Are you saying that metmat.Det(det) inverted the matrix metmat (as I understand the content should be changed but not sure how)? Anyway, the update is the right thing as far as can say.