Update guides.md (#616)

The current code causes error, since it tries to access other service which is not defined in the service.
Only changes main fastJoin example, everything else might need some review.

docs/guides.md

@@ -111,18 +111,18 @@ const { fastJoin } = require('feathers-hooks-common')
 
 const postResolvers = {
   joins: {
-    author: (...args) => async post => {
+    author: (...args) => async (post, { app }) => {
       post.author = (
-        await users.find({
+        await app.services('users').find({
           query: {
             id: post.userId
           }
         })
       )[0]
     },
 
-    starers: $select => async post => {
-      post.starers = await users.find({
+    starers: $select => async (post, { app }) => {
+      post.starers = await app.services('users').find({
         query: {
           id: { $in: post.starIds },
           $select: $select || ['name']