[7.x] Updated user model var name

[innoflash]

This PR is just changing the variable name used in a policy.

Say i have App\Teacher set as user in the config/auth.php file.

When i create a policy like php artisan make:policy --model=Subject the policy methods are currently having $user as var name eg.

public function view(Teacher $user, Subject $subject){}

This PR changes the variable name to $teacher

public function view(Teacher $teacher, Subject $subject){}