Invalid default value for database.driver

[scrohde]

In the values.yaml is:

database:
  # Supports mysql, postgres, cockroachdb, sqlite3
  driver: sqlite3

However, this value resulted in a runtime error for me.
When I consulted the documentation I see the proper value is 'sqlite'.

Other things that may or may not be bugs:

1. The linked documentation states that the default driver is 'mysql', but here it is 'sqlite3'
2. The documentation says 'mssql' database is supported but it's not mentioned in the comment. The comment is either listing supported databases, or valid values for driver, but either way it's not correct.

I humbly suggest the following (I haven't verified the correctness though - i've never even used casdoor before)

database:
  # Supported databases: MySQL, MSSQL, SQLite3, CockroachDB, and PostgreSQL
  # Valid values for 'driver': mysql, sqlite, postgres (see database configuration documentation for details)
  driver: mysql

[casbin-bot]

@tangyang9464 @JalinWang @imp2002