Calculate the average rating of the true crime category.
average rating = Divide (Sum(rating(category = 'true-crime')), Count(podcast_id(category = 'true-crime')))
SELECT AVG(T2.rating) FROM categories AS T1 INNER JOIN reviews AS T2 ON T2.podcast_id = T1.podcast_id WHERE T1.category = 'true-crime'
database name:db_id music_platform_2
runs table runs Cols: run at dtype text, max row id dtype integer, reviews added dtype integer, 
podcasts table podcasts Cols: podcast id dtype text, itunes id dtype integer, slug dtype text, itunes url dtype text, title dtype text, 
reviews table reviews Cols: podcast id dtype text, title dtype text, content dtype text, rating dtype integer, author id dtype text, created at dtype text, 
categories table categories Cols: podcast id dtype text, category dtype text, 
