docs: use angle bracket syntax for enum type def

[mattrothenberg]

Howdy! I think I found a small typo in the docs re: exporting enum types. Let me know if I'm off base.

When exporting an Enum type, it's necessary to use the angle brackets (instead of parentheses).

 export const AnswerResponse = Enum('No', 'Yes')
 export type AnswerResponse = Enum<typeof AnswerResponse>
// ^^ if we use parentheses again here, we'll get a Typescript compilation error.

If there is a linked issue, mention it here.

Requirements

• Read the contribution guidelines.
• Updated docs and upgrade instructions, if necessary.

Rationale

Why is this PR necessary?

The docs aren't accurate and might confuse people (like me).