chore: fix MR template directory name
Per https://gitlab.com/help/user/project/description_templates the directory where the MR templates exist must be name merge_request_templates but we missed this with a typo: merge_request_template.
Per https://gitlab.com/help/user/project/description_templates the directory where the MR templates exist must be name merge_request_templates but we missed this with a typo: merge_request_template.