Add notice that the pty bug only applies to Capistrano 3.

According to https://github.com/seuros/capistrano-sidekiq/wiki#other.

[ci skip]

README.md

@@ -52,7 +52,7 @@ Configurable options, shown here with defaults:
     :sidekiqctl_cmd => "#{fetch(:bundle_cmd, "bundle")} exec sidekiqctl" # Only for capistrano2.5
 ```
 
-There is a known bug that prevents sidekiq from starting when pty is true
+There is a known bug that prevents sidekiq from starting when pty is true on Capistrano 3.
 ```ruby
 set :pty,  false
 ```