docs: add missing title "features/posts/postsSlice.ts" to code block

[Talos0248]

…in "Post Reaction Buttons" section

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name: 📝 Documentation Fix
about: Fixing a problem in an existing docs page

Checklist

• Is there an existing issue for this PR?
  • Nope. Figured this also falls under "sometimes the best way to start a conversation is to send a pull request"
• Have the files been linted and formatted?
  • Yes, I ran yarn lint just in case.

What docs page needs to be fixed?

https://redux.js.org/tutorials/essentials/part-4-using-data#tracking-reactions-data-in-posts

• Section: Tracking Reactions Data in Posts
• Page: Redux Essentials, Part 4: Using Redux Data

What is the problem?

The code block in the section is missing the title (path to the file to be changed)

What changes does this PR make to fix the problem?

Added the title to the code block: title="features/posts/postsSlice.ts"

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[timdorr]

Thanks!