Adjusted an example to align with the documentation section on reversing a previous migration.

[cedriccarrard]

Motivation / Background

Fixes #50189.
There is an error in the following section this documentation section.

There is confusion between the steps

1. Reverse the order of create_table and reversible.
2. Replace create_table with drop_table.
3. Finally, replace up with down and vice-versa.

and the example below

I suggest modifying the example.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[Leo-U]

Modifying the example seems like it might be ok, at least to me. But I'm thinking maybe someone removed it for a specific reason, and without knowing what that reason was, maybe simply adding back in the 'create_table' isn't the solution? Idk, what do you think?

[cedriccarrard]

@Leo-U I think it's just an oversight when updating the documentation, what do you think @adrianna-chang-shopify ?