Fix incorrect primary structure.sql filename in the case of multi-database setup

[RKushnir]

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Hi there, we just introduced a secondary database to our Rails 6 app and were surprised to see db:migrate fail with this error:

Errno::ENOENT: No such file or directory @ rb_sysopen - db/primary_structure.sql
…/activerecord-clean-db-structure/lib/activerecord-clean-db-structure/tasks/clean_db_structure.rake:25:in `read'

Here's a little fix to get past this bummer.