fix: improve completions help formatting

[caarlos0]

did some improvements on the completions docs so they look a bit better if exported as markdown (and still look good on plain text):

• indent the example command lines so they are rendered as code blocks
• removed the $ prefix so they are easier to copy/paste
• standarized the Linux and macOS subsections when present

example:

• https://nfpm-m71s1p3z3-goreleaser.vercel.app/cmd/nfpm_completion_bash/ (without this patch)
• https://nfpm-nmv190dyc-goreleaser.vercel.app/cmd/nfpm_completion_bash/ (with this patch)

[CLAassistant]

All committers have signed the CLA.

[marckhouzam]

This is a nice improvement. Thanks @caarlos0!

[caarlos0]

Hey, what's left to get this merged? Anything else I can help?

[github-actions]

This PR is being marked as stale due to a long period of inactivity

[caarlos0]

can we merge this yet?

[afiune]

I was about to improve this as well, I wonder how can we help get this merged.

[afiune]

Hey @jpmcb 👋🏽 - Do you think you can do a quick review here, this is just improving some documentation. 🤞🏽

[umarcor]

@afiune @caarlos0 this is listed in #1496 already, and merged in #1525. See #1527 for further info.

[afiune]

This is awesome! Thank you @umarcor 🙌🏽