For movies with the keyword of "civil war", calculate the average revenue generated by these movies.
keyword of "civil war" refers to keyword_name = 'civil war'; average revenue = AVG(revenue)
SELECT AVG(T1.revenue) FROM movie AS T1 INNER JOIN movie_keywords AS T2 ON T1.movie_id = T2.movie_id INNER JOIN keyword AS T3 ON T2.keyword_id = T3.keyword_id WHERE T3.keyword_name = 'civil war'