Update part-6-performance-normalization.md

[aleksanderantropov]

To make it consistent with the previous example

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[markerikson]

We actually want to be using the "builder callback" syntax here, not the "object lookup" form. If I missed converting one of the examples to the "builder callback" form, please file a PR to fix that one instead. Thanks!

[markerikson]

Yeah, looks like I did forget to update the notificationsSlice one.

I'll reopen this - can you change the PR to fix that one instead?

[aleksanderantropov]

Added another commit with the changes

[markerikson]

I apparently updated this elsewhere at some point.