AWS Postgres setting pg_trgm.word_similarity_threshold

Trying to set pg_trgm.word_similarity_threshold on an RDS postgres instance (postgres 12 if it matters):

ALTER SYSTEM SET pg_trgm.word_similarity_threshold = 0.3; 

I get the error: must be superuser to execute ALTER SYSTEM command

I don’t see this in the parameter groups even though pg_trgm is a supported extension. Is there something I’m missing?