Pluralize backrefs

https://github.com/bio2bel/miRTarBase/blob/835330f25a9e981c8359fee3257f8c8c79beca55/src/bio2bel_mirtarbase/build_db.py#L52

Since this is a one-to-many relationship, the back-reference points to multiple "interactions" so we should name this variable with a plural