chore: replace dev-dependencies by dev-dependencies

[guissalustiano]

Hi! I was trying to build the repo for the first time and noticed these warnings:

warning: /home/[...]/diesel/examples/mysql/getting_started_step_1/Cargo.toml: `dev_dependencies` is deprecated in favor of `dev-dependencies` and will not work in the 2024 edition
(in the `diesel_demo_step_1_mysql` package)
warning: /home/[...]/diesel/examples/postgres/getting_started_step_3/Cargo.toml: `dev_dependencies` is deprecated in favor of `dev-dependencies` and will not work in the 2024 edition
(in the `diesel_demo_step_3_pg` package)
# ...

I double-checked in the cargo repo.

And looks simple, so I'm fixing these here

Thanks!