Documentation description adjust

[RafaelloLollipop]

Hey,

thanks for great work here! Found some small problem that make a small mess in my implementation:)

provide cert path wtih MONGODB_CERT - is not working for me

Some examples in docs like:

certificate = pathexpand("~/.mongodb/ca.pem")

also are invalid.

Parameter always need file() like in one example:

certificate = file(pathexpand("path/to/certificate/ca.pem"))

So probably overall text:

[certificate](https://registry.terraform.io/providers/Kaginari/mongodb/latest/docs#certificate) - (Optional) Path to a directory with certificate files for connecting to the Docker host via TLS. I. If the path is blank, the MONGODB_CERT will also be checked.

Need some small adjustments - content vs path :)