docs: add missing constant in code example (#12870)

Co-authored-by: Thang Vu <[email protected]>

Author: PieterDePauw

docs/pages/guides/pages/signin.mdx

@@ -133,6 +133,8 @@ import { redirect } from "next/navigation"
 import { signIn, auth, providerMap } from "@/auth.ts"
 import { AuthError } from "next-auth"
 
+const SIGNIN_ERROR_URL = "/error"
+
 export default async function SignInPage(props: {
   searchParams: { callbackUrl: string | undefined }
 }) {