Remove an incorrect comment for find_by example

Dynamic finder methods (`DynamicMatchers`)  has not been previously
deprecated.
AFAIK, Rails 6.0 has also no plans to deprecate it.
https://github.com/rails/rails/blob/v6.0.3.2/activerecord/lib/active_record/dynamic_matchers.rb

So, it seems that this was written incorrectly when I refer to the
following.

- #220
- #208

Therefore this PR removes the incorrect example comment for `find_by`.

Author: koic

README.adoc

@@ -831,7 +831,6 @@ User.where(first_name: 'Bruce', last_name: 'Wayne').take
 
 # bad
 User.find_by_email(email)
-# bad, deprecated in Active Record 4.0, removed in 4.1+
 User.find_by_first_name_and_last_name('Bruce', 'Wayne')
 
 # good