docs: update keyof() ZodEnum type to the v4 form

[patrickwehbe]

The .keyof() examples show the result type as ZodEnum<["name", "age"]>, which is the Zod 3 form where ZodEnum's parameter was a string tuple.

In Zod 4 ZodEnum takes an enum-like object instead (EnumLike = Readonly<Record<string, EnumValue>>), and keyof() returns ZodEnum<util.ToEnum<keyof Shape & string>>. For Dog with keys name and age that resolves to ZodEnum<{ name: "name"; age: "age" }>, so this updates both the Zod and Zod Mini examples to match.

Docs-only change.