Small improvements of instructions for Raspberry Pi 4 Model B

[yuiseki]

Summary

• This pull request does small improvements of instructions for Raspberry Pi 4 Model B, written by myself in the past.

  • 6

• Fix markdown syntax
• The description of connecting a network cable was missing, so I added it.
• Added minimal instructions for those unfamiliar with vim.

[ShogoHirasawa]

Thank you for brilliant contribution. This pull request is useful for beginner！

[ShogoHirasawa]

@yuiseki Recently, We updated wiki page. in the future, We want to merge your description into this wiki page. if you have some idea or request, Please do comment on issue!