#2718 Updated Custom Schemas documentation example to use type narrowing

[mitchgollub]

Hi,

I'm interested in updating the Custom Schemas example in the documentation. The function type takes in an unknown, so the example casts the value to a string in order to operate on it with a RegExp

const px = z.custom<`${number}px`>((val) => {
  return /^\d+px$/.test(val as string);
});

I want to propose using Type Narrowing. It's helped me catch errors from validating a non-expected type, and I believe it would help others as well as a good practice with interacting with the unknown type.

const px = z.custom<`${number}px`>((val) => {
  return typeof val === "string" ? /^\d+px$/.test(val) : false;
});

This is a PR for issue #2718

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[JacobWeisenburger]

Thanks @mitchgollub