minor formatting changes [ci skip]

* add newline for display the fenced code block
* add "#" in the comments section

activerecord/CHANGELOG.md

 *   Added `ActiveRecord::Relation#left_outer_joins`.
 
     Example:
+
         User.left_outer_joins(:posts)
-        => SELECT "users".* FROM "users" LEFT OUTER JOIN "posts" ON
-           "posts"."user_id" = "users"."id"
+        # => SELECT "users".* FROM "users" LEFT OUTER JOIN "posts" ON
+             "posts"."user_id" = "users"."id"
 
     *Florian Thomas*