Update parameters.py in vleptoquark_te_tmu_UFO and vleptoquark_tta_UFO
Changes 'mt' to 'MT' in parameters.py for vleptoquark_te_tmu_UFO and vleptoquark_tta_UFO which resolves a bug preventing use. This is the issue discussed in this JIRA ticket https://its.cern.ch/jira/browse/AGENE-1900.