docs: fix install & add z.instanceof

[THEtheChad]

Addresses the installation mismatch: https://github.com/timdeschryver/zod-fixture/issues/68

Also adds documentation for the use of z.instanceof (we can't directly support it).

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Name	Status	Preview	Updated (UTC)
zod-fixture	✅ Ready (Inspect)	Visit Preview	Jul 18, 2023 2:32am

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:tada: This PR is included in version 2.1.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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