Update oauth-with-pkce-flow-for-ssr.mdx (#20071)

I moved the headers declaration outside of the if statement code block, as in the example, we are required to provide these headers to the redirect if the code fails.

apps/docs/pages/guides/auth/server-side/oauth-with-pkce-flow-for-ssr.mdx

@@ -156,11 +156,10 @@ export async function loader({ request }: LoaderFunctionArgs) {
   const requestUrl = new URL(request.url)
   const code = requestUrl.searchParams.get('code')
   const next = requestUrl.searchParams.get('next') || '/'
+  const headers = new Headers()
 
   if (code) {
     const cookies = parse(request.headers.get('Cookie') ?? '')
-    const headers = new Headers()
-
     const supabase = createServerClient(process.env.SUPABASE_URL!, process.env.SUPABASE_ANON_KEY!, {
       cookies: {
         get(key) {