fix: default table name

[kilosonc]

Signed-off-by: closetool c299999999@qq.com

Fix: #78

[d1ss0nanz]

Cross posting from #78 since migration code is missing.

Ok, so up to at least 3.0.3, in some code paths, the created table is named "casbin_rules".
NewAdapterByDB() is behaving this way in 3.0.3.

Now, when updating to a more recent version, gorm-adapter is creating a new table "casbin_rule" besides the fact that there already exists a table "casbin_rules".

All authorization attempts silently fail, since the group membership data is in the old table.

I would suggest that you document the breaking change, so one doesn't have to understand how Gorm derives table names from struct names, to understand what's going on.

Optionally, there should be migration code that checks for a table "casbin_rules" and moves the data over when creating "casbin_rule".

[kilosonc]

@d1ss0nanz Thinks for your suggestions. In my opinion, althought support data migration is quite simple, that's not a good choice to add DML in code. So I'll emphasize this change in readme.

[kilosonc]

@d1ss0nanz @hsluoyz plz review