Update working-with-associations.rst

There are 2 many-to-many associations between the users and comments. Both use another join-table. The join-tables user_favorite_comments and user_read_comments must be specified. Otherwise the default "user_comment" is taken twice. See https://groups.google.com/forum/#!topic/doctrine-user/Kti36_n6490 and https://groups.google.com/forum/#!topic/doctrine-user/TYwafhgYiSU