Update active_record_basics.md

Modified "# Find all books with a given an author, sort by created_at in reverse chronological order."
to make it correct and more clearer in meaning.

Change: "# Find all books by a given author, sort by created_at in reverse chronological order."

Author: rasheedmhd

guides/source/active_record_basics.md

@@ -501,7 +501,7 @@ SELECT "books".* FROM "books" WHERE "books"."id" = ? LIMIT ?  [["id", 42], ["LIM
 ```
 
 ```ruby
-# Find all books with a given an author, sort by created_at in reverse chronological order.
+# Find all books by a given author, sort by created_at in reverse chronological order.
 Book.where(author: "Douglas Adams").order(created_at: :desc)
 ```