Fix error on route for Next JS tutorial (supabase#18686)

Fix error on route Co-authored-by: Terry Sutton <saltcod@gmail.com>

Author: Esteban Dalel R

apps/docs/pages/guides/getting-started/tutorials/with-nextjs.mdx

@@ -358,7 +358,7 @@ import { createRouteHandlerClient } from '@supabase/auth-helpers-nextjs'
 import { cookies } from 'next/headers'
 import { NextResponse } from 'next/server'
 
-export async function POST(req) {
+export async function GET(req) {
  const cookieStore = cookies()
  const supabase = createRouteHandlerClient({ cookies: () => cookieStore })