Merge pull request rails#42175 from OuYangJinTing/Fix-docs-of-active_record_querying-select_all

[ci-skip] fix docs for ActiveRecord select_all

Author: kamipo

guides/source/active_record_querying.md

@@ -1992,7 +1992,7 @@ This method will return an instance of `ActiveRecord::Result` class and calling
 object would return you an array of hashes where each hash indicates a record.
 
 ```irb
-irb> Customer.connection.select_all("SELECT first_name, created_at FROM customers WHERE id = '1'").to_hash
+irb> Customer.connection.select_all("SELECT first_name, created_at FROM customers WHERE id = '1'").to_a
 => [{"first_name"=>"Rafael", "created_at"=>"2012-11-10 23:23:45.281189"}, {"first_name"=>"Eileen", "created_at"=>"2013-12-09 11:22:35.221282"}]
 ```