Fix prefix unary increment operator usage in Update chapter-07.md

[hrxn]

PR Summary

Invalid syntax in use for example here

PR Checklist

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[sdwheeler]

Thanks for the fix.

[learn-build-service-prod]

Learn Build status updates of commit 0084bb8:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/lang-spec/chapter-07.md	✅Succeeded	View (>=powershell-5.1)

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[hrxn]

Oh, wait.

I just saw that there are other instances of this wrong operator syntax, at least in this example block. Maybe even more in that chapter/file.

I did not fix any of these, only this one which I immediately noticed.

[hrxn]

But apparently only two more instances.
I'll make a new PR.