add-table-name should output `table_name` as table instead of string

[yaziciahmet]

I am using diesel 2.0.2.

Considering I have a table named article, the outputted table_name when used with --add-table-name option is string, eg. #[diesel(table_name = "article")]. But it should actually be something like #[diesel(table_name = crate::schema::article)].