fix errors in buildClerkProps docs (#2798)

Author: HassenHichri

docs/_partials/nextjs/build-clerk-props.mdx

@@ -6,7 +6,7 @@ import { GetServerSideProps } from 'next'
 
 export const getServerSideProps: GetServerSideProps = async (ctx) => {
   // Use `getAuth()` to access `isAuthenticated` and the user's ID
-  const { isAuthenticated } = getAuth(ctx.req)
+  const { isAuthenticated, userId } = getAuth(ctx.req)
 
   // Protect the route by checking if the user is signed in
   if (!isAuthenticated) {