Avoid unary negation of unsigned in FormatValue
Avoid unary negation of unsigned in FormatValue

Summary:
[Folly] Avoid unary negation of unsigned in `FormatValue`, which some compilers may warn against.

```
folly\format-inl.h(452): error C4146: unary minus operator applied to unsigned type, result still unsigned
```

Reviewed By: Orvid

Differential Revision: D14141989

fbshipit-source-id: 3bff06fe09fd4ec22d93b72a74f086ad9c576692

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