fix: use correct naming for .env variable db name

[relu91]

Substituted the variable name for the database name in the .env file from ZION_DB_TYPE to ZION_DB_DATABASE.

fix #59

Question:

Shall I also remove ZION_DB_TYPE ?

[hyperloris]

Yes, let's also remove DB_TYPE.

[ivanzy]

DB_DATABASE is kind of counter-intuitive, since DB is short for database. Also, it as some side-effects as config.database.database.

[relu91]

DB_DATABASE is kind of counter-intuitive, since DB is short for database. Also, it as some side-effects as config.database.database.

See https://github.com/vaimee/zion/issues/59#issuecomment-1582062077