Fix README about bundle_bins

Fixes: capistrano#47

README.md

@@ -33,7 +33,7 @@ By default, the plugin adds `bundle exec` prefix to common executables listed in
 
 You can add any custom executable to this list:
 ```ruby
-set :bundle_bins, fetch(:bundle_bins, []).push %w(my_new_binary)
+set :bundle_bins, fetch(:bundle_bins, []).push 'my_new_binary'
 ```
 
 Configurable options: