docs: fix bidirectional association examples in v7 upgrade guide

[medgardo]

Fixed bidirectional examples that were inconsistent.

• The model Country and Countries was used. I updated all of the models in the example to use singular names.
• Updated association aliases to be plural names citizen -> citizens as recommended for many-to-many associations
• Fixed an error associating User to User (User.belongsToMany(User, { foreignKey: 'user_id' });) instead of to Country as further explained in the example

Thanks for the work on v7, it's looking great. I'd be happy to make changes if I misunderstood the intent of the examples here.