Fix markdown format in deprecation_date.md

[syou6162]

What are you changing in this pull request and why?

There were some places where the markdown output did not seem to be as intended, so I fixed them.

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• I have added checklist item(s) to this list for anything anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."
• The content in this PR requires a dbt release note, so I added one to the release notes page.

[welcome]

Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
We use Markdown and some HTML to write the dbt product documentation. When writing content, you can use our style guide and content types to understand our writing standards and how we organize information in the dbt product docs.
We'll review your contribution and respond as soon as we can. 😄

[vercel]

@syou6162 is attempting to deploy a commit to the dbt-labs Team on Vercel.

A member of the Team first needs to authorize it.

[mirnawong1]

hey @syou6162 , great catch! thanks for opening this pr up to fix this up - looking forward to future contributions ✨ !