Update to Custom Schemas example in Documentation

[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;
});

Please let me know if this is a welcome change. I can make a fork of the repo and create a PR!

[JacobWeisenburger]

Sounds good to me. Please make the PR and I'll take a look.

[mitchgollub]

Thank you for the opportunity to contribute @JacobWeisenburger! I've made my PR here: #2778