fix some grammar mistakes (#49)

Author: kerolloz

rails6/en/chapter04-athentification.adoc

@@ -321,7 +321,7 @@ $ mkdir test/controllers/concerns
 $ touch test/controllers/concerns/authenticable_test.rb
 ----
 
-As usual, we start by writing our tests. In this case, our `current_user` method will search for an user by the authentication token in the HTTP header `Authorization`. The test is quite basic:
+As usual, we start by writing our tests. In this case, our `current_user` method will search for a user by the authentication token in the HTTP header `Authorization`. The test is quite basic:
 
 [source,ruby]
 .test/controllers/concerns/authenticable_test.rb

rails6/en/chapter06-improve-json.adoc

@@ -1,7 +1,7 @@
 [#chapter06-improve-json]
 = Building JSON
 
-In the previous chapter we added products to the application and built all the necessary routes. We have also associated a product with an user and restricted some of actions of `products_controller`.
+In the previous chapter we added products to the application and built all the necessary routes. We have also associated a product with a user and restricted some of actions of `products_controller`.
 
 Now you should be satisfied with all this work. But we still have a lot of work to do. Currently we have a JSON output that is not perfect. JSON output looks like this:
 

rails6/en/chapter07-placing-orders.adoc

@@ -25,7 +25,7 @@ $ git checkout -b chapter07
 
 == Modeling order
 
-If you remember associations model, `Order` model is associated with users and products at the same time. It is actually really simply to achieve this in Rails. The tricky part is when comes to serializing this objects. I'll talk more about this in a next section.
+If you remember associations model, `Order` model is associated with users and products at the same time. Actually, It is really simple to achieve this in Rails. The tricky part is when it comes to serializing this object. I'll talk more about this in a next section.
 
 Let’s start by creating the order model with a special form:
 

rails6/en/chapter09-optimization.adoc

@@ -551,7 +551,7 @@ Now let's make a request with cURL. I remind you we must obtain an authenticatio
 $ curl -X POST --data "user[email][email protected]" --data "user[password]=locadex1234"  http://localhost:3000/api/v1/tokens
 ----
 
-NOTE: "[email protected]" corresponds to an user created in my application with the _seed_. In your case, it will probably be different from mine since we used the Faker library.
+NOTE: "[email protected]" corresponds to a user created in my application with the _seed_. In your case, it will probably be different from mine since we used the Faker library.
 
 With the help of the token obtained, we can now make a request to access the products