Include `**/factories.rb` in the inspection target

[r7kamura]

According to the factory_bot documentation, the default FactoryBot.definition_file_paths also includes factories.rb.

The load order is controlled by the FactoryBot.definition_file_paths attribute. The default load order is:

1. factories.rb
2. test/factories.rb
3. test/factories/**/*.rb
4. spec/factories.rb
5. spec/factories/**/*.rb

• https://github.com/thoughtbot/factory_bot/blob/cf3f21fcbbccbc40849c74a84eeb0bb0bd6c7606/docs/src/ref/find_definitions.md?plain=1#L9-L13

Therefore, it would be reasonable to include this file in the inspection target.

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• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [ ] Added tests.
• [x] Updated documentation.
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• [ ] Added the new cop to config/default.yml.
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