Update GetCrossSectionAMITool.py to comply with pyami's fixed typo

So far pyami had a typo in the api, which apparently changed now:

Py:GetCrossSectionAMICRITICAL The reason for failure was: pyAMI exception: invalid field `generator_filter_efficienty`, not in [`ami_status`, `ami_tags`, `atlas_release`, `beam`, `completion`, `conditions_tag`, `cross_section`, `dataset_number`, `ecm_energy`, `events`, `generator_filter_efficiency`, `generator_name`, `generator_tune`, `geometry`, `in_container`, `job_config`, `ldn`, `modified`, `nfiles`, `pdf`, `period`, `physics_comment`, `physics_short`, `prodsys_status`, `production_step`, `project`, `requested_by`, `responsible`, `run_number`, `stream`, `total_size`, `transformation_package`, `trash_annotation`, `trash_date`, `trash_trigger`, `trigger_config`, `type`]

I fixed the typo in the Tool. Tagging @jbeaucam