Update lesson plan template

[JulieKuehl]

Bring into alignment with the latest lesson plan template found at https://github.com/wptrainingteam/repo-template/

[jessecowens]

@showw

[megane9988]

I think this issue solved by these pull requests

https://github.com/wptrainingteam/introduction-to-css/pull/22/files
https://github.com/wptrainingteam/introduction-to-css/pull/21/files

@showw
@JulieKuehl