docs: missing comma in custom-strategies example (#13701)

Fix missing comma at custom-strategies example

docs/authentication/custom-strategies.mdx

@@ -49,7 +49,7 @@ export const Users: CollectionConfig = {
     strategies: [
       {
         name: 'custom-strategy',
-        authenticate: ({ payload, headers }) => {
+        authenticate: async ({ payload, headers }) => {
           const usersQuery = await payload.find({
             collection: 'users',
             where: {
@@ -66,7 +66,7 @@ export const Users: CollectionConfig = {
             // Send the user with the collection slug back to authenticate,
             // or send null if no user should be authenticated
             user: usersQuery.docs[0] ? {
-              collection: 'users'
+              collection: 'users',
               ...usersQuery.docs[0],
             } : null,