Fix incorrect identifier quoting [ci skip]

activerecord/lib/active_record/relation/query_methods.rb

@@ -243,10 +243,10 @@ def _select!(*fields) # :nodoc:
     # Allows you to change a previously set select statement.
     #
     #   Post.select(:title, :body)
-    #   # SELECT `posts.title`, `posts.body` FROM `posts`
+    #   # SELECT `posts`.`title`, `posts`.`body` FROM `posts`
     #
     #   Post.select(:title, :body).reselect(:created_at)
-    #   # SELECT `posts.created_at` FROM `posts`
+    #   # SELECT `posts`.`created_at` FROM `posts`
     #
     # This is short-hand for <tt>unscope(:select).select(fields)</tt>.
     # Note that we're unscoping the entire select statement.

guides/source/active_record_querying.md

@@ -819,19 +819,19 @@ Post.select(:title, :body).reselect(:created_at)
 The SQL that would be executed:
 
 ```sql
-SELECT `posts.created_at` FROM `posts`
+SELECT `posts`.`created_at` FROM `posts`
 ```
 
 In case the `reselect` clause is not used,
 
 ```ruby
-Post.select(:title, :body)
+Post.select(:title, :body).select(:created_at)
 ```
 
 the SQL executed would be:
 
 ```sql
-SELECT `posts.title`, `posts.body` FROM `posts`
+SELECT `posts`.`title`, `posts`.`body`, `posts`.`created_at` FROM `posts`
 ```
 
 ### `reorder`