fix: remove unnecessary parentheses in draft mode API (#75153)

## What?
Updates the draft mode documentation to fix incorrect method calls in
the code examples.

## Why?
The code examples were showing incorrect syntax for enabling and
disabling draft mode, using `draft()` instead of accessing the methods
directly on the `draft` object.

## How?
Removes the unnecessary parentheses when calling `enable()` and
`disable()` methods on the draft mode object in the documentation
examples.

Fixes vercel/feedback#82345

docs/01-app/04-api-reference/04-functions/draft-mode.mdx

@@ -54,7 +54,7 @@ import { draftMode } from 'next/headers'
 
 export async function GET(request: Request) {
   const draft = await draftMode()
-  draft().enable()
+  draft.enable()
   return new Response('Draft mode is enabled')
 }
 ```
@@ -64,7 +64,7 @@ import { draftMode } from 'next/headers'
 
 export async function GET(request) {
   const draft = await draftMode()
-  draft().enable()
+  draft.enable()
   return new Response('Draft mode is enabled')
 }
 ```
@@ -80,7 +80,7 @@ import { draftMode } from 'next/headers'
 
 export async function GET(request: Request) {
   const draft = await draftMode()
-  draft().disable()
+  draft.disable()
   return new Response('Draft mode is disabled')
 }
 ```
@@ -90,7 +90,7 @@ import { draftMode } from 'next/headers'
 
 export async function GET(request) {
   const draft = await draftMode()
-  draft().disable()
+  draft.disable()
   return new Response('Draft mode is disabled')
 }
 ```