[Docs] Tidy up the Modal tutorial

[daniel-prefect]

This is a follow-up to #17345. We decided to address style and formatting in a separate PR rather than slowing down our external contributor.

Preview: https://prefect-bd373955-tidy_the_modal_tutorial.mintlify.app/v3/tutorials/modal

A few of the tweaks I made (not an exhaustive list):

• Fix capitalization (e.g. GitHub, Modal, Prefect).
• Replace 'we' and 'our' with 'you' and 'your'.
• Use ventilated prose.
• Assign links the name of the linked page instead of "here" or "this".
• Utilize numbered lists in places with tightly coupled steps.
• Put terminal commands on a new line (as opposed to being inline with prose) so that it's more obvious what you're supposed to run.
• Show the proper name of UI elements and format them in bold.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

[zzstoatzz]

this is great! thanks for the cleanup @daniel-prefect 🎉