Fix documentation for PostgreSQL `DB_TYPE` value

[gbannerman]

This change fixes the documentation for the PostgreSQL environment variable values. Previously the documentation stated that the expected DB_TYPE value when using PostgreSQL was "pgsql", but the DBType enum actually expects a name of "postgres" which then maps to a value of "pgsql".

It also fixes a small formatting mistake in the README.md file.

[gbannerman]

Thanks for this image, it works really well and is nice and clearly documented. I noticed a slight mistake when using it with Postgres though, so thought I'd fix it.

You can see that the enum value expects "postgres": https://github.com/NathanVaughn/webtrees-docker/blob/435a06bdb5e850d8e282ec52b2bfa15fa535242b/docker-entrypoint.py#L131 https://github.com/NathanVaughn/webtrees-docker/blob/435a06bdb5e850d8e282ec52b2bfa15fa535242b/docker-entrypoint.py#L14

[NathanVaughn]

Thanks for pointing this out! I corrected it in the opposite direction of expecting pgsql per the documentation, to not break backwards compatibility. I missed this when I rewrote the Python launch script.