docs: fix code example typo in essentials/part-6-performance-normaliz…

[robertgr991]

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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?
  • link issue here
• Have the files been linted and formatted?

What docs page needs to be fixed?

• Section: Tutorials > Redux Essentials
• Page: Performance, Normalizing Data, and Reactive Logic > Showing Toasts for New Posts

What is the problem?

Argument startListening of addPostsListeners function should be startAppListening.

What changes does this PR make to fix the problem?

Changes the argument name to the one used in the function body and in other sections of this page.

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[EskiMojo14]

thanks!