docs(README): fix code sample and describe forPrimitives

[forivall]

While resolving a different issue, I didn't immediately understand the forPrimitives option, and it wasn't documented anywhere, so I figured it out by looking at the one place where it's used, and looking up the documentation for squire and noticed a glaring issue on the README - in the readme, createFilter is used, but no such function exists, and createFactory is the correct function.

Other notes

I also would love to have custom operators to have better typing, like the following snippet:

type JSONSchemaQuery<T> = BuildMongoQuery<T, { toplevel: { $jsonSchema: import('json-schema').JSONSchema4 }}>
type JSONSchemaFilter = <
  T = Record<string, unknown>,
  Q extends JSONSchemaQuery<T> = JSONSchemaQuery<T>
>(query: Q) => ThingFilter<T>
const customGuard = createFactory({
  $jsonSchema,
}, {
  jsonSchema
}) as JSONSchemaFilter;

[stalniy]

thank you for contribution!