demotion-of-integers.md: Fix example

[Alcaro]

-2 isn't 0xFE. (char)-2 is 0xFE, but IMO adding a type cast would distract from the point of that page; instead, let's switch to integers' default size.

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[TylerMSFT]

Thank you for this fix. Excellent.

[TylerMSFT]

#sign-off

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