In general you should try to prefix your function names with your project or company name e.g. instead of people_posttype you could prefix with it e.g. it_ people_post_type
The reason this is important is it helps a bit to avoid errors, namespace collisions, e.g. if you install another plugin that has a function name people_post_type an error would occur. Although it seems unlikely that this may happen it's considered best practice to prefix all functions with your company or project name.
In general you should try to prefix your function names with your project or company name e.g. instead of people_posttype you could prefix with it e.g. it_ people_post_type
The reason this is important is it helps a bit to avoid errors, namespace collisions, e.g. if you install another plugin that has a function name people_post_type an error would occur. Although it seems unlikely that this may happen it's considered best practice to prefix all functions with your company or project name.
https://github.com/redacademy/it-winter-2018/blob/cf684effeca6adc3f68a3c56649aacfb086c7b23/plugins/it-functionality/lib/functions/post-types.php#L12